As pamtbaau highlights in the other topic, solutions to highlighting whether themes and plugins are compatible with the current version of Grav core have been proposed and discussed in various GitHub-issues and -discussions, as well as through the various iterations of the forum and chat-channels, for years.
I wholeheartedly agree that this is a necessity for many users, even though most of these extensions are actually working even without updates for years. Grav’s Core API has been that reliant. Some of this would be supported through the long-promised and -awaited GPM 2.0, but that has yet to see the light of day.
I would rather propose to come at a solution from the other side: The community, especially those without as many years of developer-experience or familiarity with Grav’s codebase, should come together in creating and maintaining a channel for review and quality assurance of which parts of Grav’s ecosystem works at present. From my knowledge of the Core Dev Team, there is not excess capacity for community-building.
This could take many forms, but a simple repository that frequently maintains the sorts of tests from Theme/plugin repository cleanup and quality control, retains knowledge of the peculiarities of versioning, and other information about the ecosystem would greatly help. It would also pair especially well with a blog where contributors could highlight experiences from testing themes and plugins, and a set of pages documenting possible solutions.
Which is all to say, the community around Grav and its ecosystem would greatly benefit from a community-driven solution. This would also make it a great deal easier for the Core Dev Team to perform maintenance of the list of themes and plugins.