I found the answer myself:
I created an empty file “none.png” and use that as target:
<key>Icon</key>
<string>none.png</string>
It is not even necessary to create an empty file, just any non-empty string seems to suppress the message.
I’m a little on the slow side today 
Just don’t include the Icon key or the string at all.
That is how it should be. You’ll get the message until the next version.
But yes, if it references a non-existent file, that will work, but down the track, that might complain.
Yes, indeed, this is the clean solution. Thanks for the hint. As said, really on the slow side today 
Thanks for the quick response and update Tom, and for the KM8 version.
I will download and install.
Well, since my plugin apparently has found its first user — and, on top of that, one who is reporting issues — it goes without saying that I will do my best to care for that user 
Thanks, BTW, for your update note in the original plugin post.
In the actual update post I have replaced your yellow "Update" image by actual text, because I don't think it is a good idea to replace text with images, even if it's only one word.
But, to maintain the emphasis, I added a nice icon. It's from the free version of Font Awesome, downloadable as image data from here.
That's fine. It is your post and should reflect your style and preferences.
I still like the "updated" image because it is a replacement for rich text that is not available in the forum. IMO, the yellow is a better eye-catcher. 
For decades now, we have seen the word "new" as a graphic image to call attention to something, better than just a text word would do.
I'm not a big fan of the bell icon, but I'll have to take a look at all of the icons to see if there are any of interest. Thanks for sharing.
That's not what I meant. Rich Text is still text. In a text (or Rich Text) context it's a matter of "style and preferences", agreed. But replacing text with images actually removes the text from the article. When the article is parsed the word "update" will simply disappear, because it is an image, i.e. non existent as text. I guess this has also implications for accessibility.
Adding an illustrative icon is another thing, because the essential thing (the text) is still there and readable. The icon is optional.
I don't want to tell you how to write your posts. It's just to explain why I prefer to not do that.
Good point, one that I had not considered.
Any ideas of how to produce "updated" with yellow highlight that is text, in this forum?
Nope. I'm note very familiar with Discourse, but I don't think this is possible. We would need the background-color CSS attribute for that, no? Or the HTML5 <mark> tag…
It seems it can be done on the admin side by customizing the CSS:
I figured it out, I think. 
Put "alt" and "title" in the image src statement:
<img src="/uploads/default/original/2X/9/9fa034801953dfe7d1745afcb7087c7530a06e23.gif" width="70" height="17" alt="updated" title="updated"> 2017-09-26 17:41 CT
and you get this:
2017-09-26 17:41 CT
and the forum search finds it:
Unfortunately, the browser search does not find it, but I can live with that.
also, I think the "alt" tag will be picked up by accessibility tools.
Well, yes, this is a workaround to make it a bit more accessible. But the proper way would be the background-color (via CSS).
Agreed, but not available in Discourse forums.
EDIT:
Seems to be that it is fully accessible. Found by forum search and accessibility tools. What am I missing?
Are you sure? The CSS can be customized, no? (See my search link above.)
So, a tag like <mark>, or a custom tag, should be possible; unless I'm misunderstanding how that thingy works
That's already a good thing. However a caption or an "alt" text is still outside of the text context.
Until/IF Discourse forum supports CSS by the user in a post, I think this is the best I can do.
The Discourse owners/devs are not a very friendly bunch, so I don’t think I’ll even request it. It was already noted as an issue/request years ago.
Bringing up this topic again.
Thanks a lot Tom for the KM to set the text notification position. I found it after i wrote the applescript myself... 
but i like your action more.
Is there a way to tell the window before it shows, up instead of afterwards? The position must be stored somewhere within the KM preferences or logs or whatever, as it might (...) popup on a the place where you moved it to when launched before.
Hey @Fokke,
Nothing sanctioned.
I wouldn't want to hack Keyboard Maestro's prefs while it was running either...
-Chris
@ccstone yeah, understood. I just looks a bit funny that my Text Screen appears and then moves. But this is how it is i presume.
Presently.
If you don't like it be sure to make a feature request to Peter.
Personally I think it would be nice to be able to specify a window name, position, and size.
-Chris