This reminds me of a message posted throughout the NVDA Community a few months ago:
While you can tell NVDA that an add-on is compatible by editing its manifest, not all add-ons will work after you do this. For example, if you think an add-on should be compatible yet is written in Python 2, it will not work because Python itself cannot run add-on modules. Most importantly, even if an add-on is written in Python 3, if it uses removed NVDA features, it will not run properly.
The amount of effort required to get an add-on supported and running well depends on how the add-on source code is structured and features in use. For example, I spent the last 24 hours just updating Clock add-on to use newer NVDA technologies and to clean up the code (time spent includes testing changes), and I plan to optimize the add-on further for speed (this takes more time to do so because it involves techniques not many add-on developers use; one such technique is Python bytecode disassembly).
At this time, I would like to suggest that you tell us names of add-ons that are not working with NVDA 2021.1, and the community will find out if updates are available or someone can maintain them. Also, if you haven't, I advise installing Add-on Updater so you can receive latest add-on updates as they become available.