I'm guessing that onepassword7://open_and_fill requires 1Password version 7 to work. I'm unclear why this would not have thrown an error in Keyboard Maestro 8.
Indeed, I would definitely agree. I've been using 1Password.com for awhile now, and think it is excellent. However, if you want, you can still buy 1Password rather than sign up for a subscription.
I don't use 1Password. But I went to their website and it says you can choose between the subscription model and the purchase model for macOS. https://1password.com/downloads/mac/ Do you have an old version that you don't want to pay to get a current version? I don't like paying for anything either, except for Keyboard Maestro, so I use macOS Keychain, which I thought was similar functionally. Personally I'm paranoid about trusting anyone with my passwords. So I went and researched 1Password, and paranoia doesn't leave early, but their product doesn't look too bad, especially if you use it in standalone mode, which I doubt anyone does.
I upgraded to 1PW7
I'm not a fan of the subscription model, but this was the path of least resistance. They do have a stand-alone version, sans online features.
Just as an aside… the Safari extension from 1Password 6 will not work with Safari 13. Apple has completely changed the extension format in a way which made all of the old ones incompatible. You pretty much have to get them through the Mac App Store now. FWIW/FYI.