FWIW, I've been running Catalina on one iMac since it was available, and have not run into any problems, including problems with KM (though I don't use KM nearly as expertly or extensively as many on the forum). I long ago dealt with 32-bit apps, as that's been suggested by Apple and others for several years.
As others in this thread noted, the key potential issue for any macOS update is if you have mission-critical apps that won't work in the new macOS version.
thank you.
I should have thought of it! thank you
Just note, everyone, that you can't install an OS that is from before a Mac was introduced. Making up an example here, but if Apple updated the Macbook Air next week, and it shipped with 10.15.6, nothing earlier could be installed on one of those, even 10.15.5.
Could it be done through virtualization? That I don't know.
very good to know
sorry, didn't see this.
Haven't used FileVault since it encrypts the disk and I thought it might have an impact in performance.
I don't think that is correct. Can you please point to a reference that confirms this?
I can't find something like an Apple support document—maybe my search terms aren't good, but nearly all the links I find are about how to move a Mac from some recent release to the preceding release. Here's one of those, though, that matches what I said above:
Apple has started shipping Mac models that used to come with Mojave pre-installed with Catalina. If your organization has blockers for Catalina (incompatible software, etc.) you may want to install Mojave on these Macs. Unfortunately, this is not so easy.
Important Notice: these instructions will only work for Mac models that can boot to Mojave. Usually a Mac requires at least the version of macOS that the model shipped with when it was introduced. As of this writing, all new Macs require at least Mojave. The exceptions are the iMac Pro (High Sierra) and the MacBook Pro 16“ and the Mac Pro (2019) which both require Catalina. You cannot use these instructions to force a Mac Pro or MacBook Pro 16” to boot to Mojave. Any new Mac models that Apple introduces from now on, will also require Catalina and cannot be downgraded to Mojave.
Source: https://scriptingosx.com/2020/01/downgrading-a-mac-that-shipped-with-catalina-to-mojave/
Taking this from another angle... The late 2012 Mac mini I use originally shipped with 10.8.1. Any OS version prior to that wouldn't include support for the hardware. Maybe there are hacks that would enable doing that, but I'm not familiar with any as it's not a practice I follow.
Another similar example, from a post titled How To Downgrade OS X Lion To Snow Leopard: The Complete Tutorial:
Before getting started, keep in mind that this guide will not work on Mac hardware introduced after the final version of Lion was released. At the time of this writing, the mid-2011 Mac Mini and the MacBook Air from the same time period are both incapable of running Snow Leopard.
As I said, this Mac mini was introduced with OS X 10.8.1. I just launched the 10.7 Lion installer and got this message:
So, using your own source, it seems clear, that as of now, you CAN downgrade all new Macs to Mojave, except for two. It can be done, it is just not easy.
Tidbits has also addressed this issue:
How to Downgrade a New Mac to Mojave from Catalina
As expected, Apple has started shipping new Macs with macOS 10.15 Catalina. We’re still hesitant to recommend upgrading to Catalina, particularly if you use Mail, since reports of email data loss continue.
==Luckily, downgrading a new Mac from Catalina to 10.14 Mojave is possible== for models other than the 16-inch MacBook Pro and the 2019 Mac Pro. Mac scripting expert (and author of the new book Moving to zsh ) Armin Briegel has published a guide explaining how to roll back a new Mac that ships with Catalina to Mojave.
==Unfortunately, the process to install Mojave isn’t straightforward, requiring a Mojave Installer USB drive.== It’s particularly fussy if your Mac is equipped with a T2 security chip since you have to set up an account before you can use Security Utility to enable booting from external drives. Regardless, you will have to erase the internal drive entirely before you can install Mojave due to Catalina’s bifurcated drive structure.
Interestingly enough, the Tidbits article points to the same source as you did.
Right, which jibes with my initial comment that "you can't install an OS that is from before a Mac was introduced." I just wanted to be certain no one reading (now or later) misunderstood the capabilities and thought they could buy any Mac at any time and install any prior OS onto it.
FWIW, I started collecting apps for the Mac in 1984, (my point being that I had almost 1000 32-bit apps, some -very- expensive) and I curretnly make my living on my machine. ( I was the first third-party developer ever hired by Apple, back in 1978.) I moved to Catalina with the latest (10.15.3). I used Go-64 to sort out my apps, and was more than a little surprised to find that, on inspection, I didn't really NEED any of them, although a few dozen were useful enough to keep around.
I installed Mojave using Parallels. (Actually, I also installed High Sierra, but eventually decided I didn't need it.) My VMs are on a fast SSD, and my machine is pretty fast too. I've discovered that a VM is more than satisfactory. In fact, it's hard to tell that it's a VM at work here.
Point of fact is that I have not yet (what - a month or so?) really found a NEED to fire it up... although I've messed with several of the apps to make sure they work.
As to Keyboard Maestro: I'm not a power user. Lots of launchings; text shortcutting; several sequences-as-shortcuts and so on. No 14-page scripts. And in that scenario, KM works just fine in Catalina. I'm on my machine 10-13 hours a day, almost 7 days a week, so if something was going to show up, I'd expect it would have by now.
This all strictly anecdotal; just based on my own personal experience. Hence you can take it FWIW, remembering that YMMV. 
Answering the original question - there are no issues with Keyboard Maestro under Catalina.
Catalina has additional security restrictions, and as well as being generally annoying, their permissions have a variety of bugs and poor behaviour, which you have to work through to ensure Keyboard Maestro has permission to do what you ask it to do. However, as far as I am aware, there are no further issues running Keyboard Maestro under Catalina once you drag the permission systems in to line.
what ??
OK, at a strict count, I had 788... you can collect a lot of cruft over 40 years
(Right now, my (64-bit) apps folder shows 754 items...)
Tracy Valleau
tracy@valleau.art
I’m still running Mojave as well, but I want to upgrade to Catalina so I can use its voice command features. (Nuance stopped upgrading Dragon Dictate a few years ago. It still works in Mojave, but has serious performance issues in apps like Safari.)
I also have a few 32-bit apps in Mojave that I still want to use, so I plan to run clone copy of Mojave when I need to use those apps. Sadly it’s only like two of them. I’ll strip down that clone copy as much as possible so I can use use the 32-bit apps.
It seems that Catalina should be stable enough by now, correct?
Jump in Bakari. Catalina is fine and the voice dictation/control work well.
KC
I have used a Mac since mid-80s, I love living on the edge, generally running betas most days on iOS and Mac. I upgraded my 2015 iMac to Catalina in December. Two weeks ago I downgraded to Mojave. First time I have EVER downgraded a system.
Tried diligently to solve the running problems but every time you squash one, 3 more show up, the permissions issues is REAL and will make your life a living nightmare, ESPECIALLY if you have more than a basic Mac setup (e.g. peripherals, drives, network, etc)
I have come to the conclusion - Catalina is the first in a line of "stupified" systems designed for the general consuming public. Think Apple's "Simple Finder" if you know what that is. The permission issues, would probably only plague you if you ventured outside Mail, Pages, and Safari. I think the goal is eventually to just make Macs expensive iOS devices replete with all the security issues and walled gardens we have now.
I have been more productive in the last 2 weeks since downgrading to Mojave, than I was the whole month of January. Makes me sad to think this might be the end of the line for me and Mac, I hope not! I keep hoping Apple will come to their senses and realize there is still a very active community of medium to advanced Mac users out their that still require the level of personal control over our devices that we are used to. Maybe just an alternate install with relaxed perms, I don't know - but this throat-shoving is not working for me.
I would call constantly appearing dialogs, confirmations, roadblock, etc to be an ISSUE with KM and in general ALL automation software in Catalina.