One you might not want to be apart of certain groups.
Two you may even want to mute that group which is a new option.
You may have selected other groups and the system reclassified to a different group that you don't like.
You may wish to change your posts groups, make it private, or just outright delete a post that was assigned to a group.
This being done by software. It takes time for the software to determine where a group or post should be located. The structure itself was likely done long ago. And the software is more suited to go through all the posts. It's why the notifications come in batches.
These notifications are there to give people a choice. Of course many may not do anything. Therefore you can ignore notifications. It really shouldn't be that much longer for most people.
Most people never had that many groups anyway. Not like me with over 7,000 groups before the updates. I literally have thousands of such notifications! 🤣
I don’t really even try to figure it out any more. I come on here read the posts that’s on my feed ( that’s not many anymore) and I reply to some. I have a few PMs sometimes I usually answer them. That’s it.