I've also noticed the fragmentation of the ecosystem. There is still some powerful stuff out there, but it is hard to find and UX has a lot of room for improvement (especially for laypeople).
Part of the fragmentation (on the extension side at least) came from Manifest V3 which required a massive re-write of logic and introduced a lot of friction for userscript managers. Many projects just died or stayed in maintenance mode since it was a big undertaking. MV3 certainly has been a pain to work with on our side.
For Greasemonkey proper (which has always only been a Firefox extension) the big pain point was Mozilla's forced migration to new extension APIs (2015: https://blog.mozilla.org/addons/2015/08/21/the-future-of-dev... ). This required a major rewrite, taking over a year, and not to add new features but rather just to not bit rot away. Then what felt like right after that, they completely deprecated classic extensions, forcing only web extensions (2017: https://blog.mozilla.org/addons/2017/02/16/the-road-to-firef... ). This required an even more thorough rewrite again, and made it not difficult but actually impossible to keep all functionality.
Greasemonkey has been stable (not abandoned, but not worked on very much!) since then. No forced MV3 yet in Firefox.
My understanding is that it's a bit more of a fragmented ecosystem now but I could be wrong.