While improving the compatibility info sounds like a great idea and I know there are been discussions about it in the past, lemme go all out and say deleting old plugins is the most absolute fucked idea I've heard of. We plugin developers are not developing Dokuwiki stuff for hire! If you are willing to change that from your own pocket, by all means, have a go at it and file the necessary issues on the trackers as well as maintaining the list of plugins and whatnot.
For example, I specifically maintain stuff up to Greebo because the Hogfather upgrades were such a mess (changes in PHP requirements plus breaking, non-retrocompatible code changes in PHP 7.4) that I opted to just skip the release, and most stuff I code is syntax anyway so there's no particular reason it shouldn't work on release N+1 if it works for release N. Besides I try to maintain a single codebase for my plugins. But it's not my responsibility to list each and every release of DokuWiki that my plugins would be compatible with. Honestly what I'm missing for the repodata table is a wildcard syntax that lets me mark my plugins as compatible by default (eg.:
compatible: __all__) and then list exceptions only (eg.: