This makes me think...
What about adding a "beta7" subtag under "Extensions", then when beta8 is out add "beta8".
Then add those subtags to extensions. We could start by having every extension having "beta7" (or 6,5,... or none)
When an existing extension is made compatible with beta8, the developer can ask a moderator to change the tag.
On an extension discussion either keep only the most recent tag (this gives an idea of whether the extension is active or not) or leave all beta tags this extension ever supported to show history.
This could be a great way for non-tech people who don't want to go to Packagist/Flagrow and just want a rough idea of the list of active extensions.
Technically of course this could somehow be automated if you can link discussions with their packagist entries then use a script too look at the Flarum version requirement. Problem is, only a handful of extensions use the "Flagrow" standard to link discussions with Packagist so doing it by hand is probably the easiest at this point.
Instead of a trash can, this doesn't throw non-updated extensions away in a corner, this just makes it easier to find extensions for the latest Flarum while leaving a change for the others to update.
What do you think ?