Apologies, I don’t think I explained my idea fully.
Yes, I totally accept that the core team doesn’t support third-party plugins. So, my proposal avoids the core team having to do any direct support or ongoing development around third-party plugins.
All tests on all plugins would be run during the upgrade process. If any fail, the upgrade is seamlessly and automatically cancelled.
This happens on self-hosted instances, not on meta. It is the self-hosting admin’s responsibility to disable failing third party plugins, and to contact the developer of those plugins.