1. Web3 hired a lot of these people and so they had less time to work on this stuff. Shame to spend that much on a dead end but eh
2. Scala died with Big Data. It is still around and all but noone care anymore, which emptied the room. It also happened that the whole Implicits experiment for polymorphism, which scala was really supposed to explore, did not pan out that well
3. Effects progressed but... Mostly out of view. Ocaml shipped them with its multicore, we are seeing good work on the academic side, you see Verse wanting them, etc. Same thing with linear types.
4. Dependent types ... Never really crossed to the realm of production. And Idris and co are mostly "complete" so it slowed down
5. Oh and monad interest, mostly fueled by scala, died slowly. Effect handlers seems to be a nicer solution in practice to most of this stuff.
6. Typescript killed a lot of the need for advanced stuff, same with python and ruby shipping their stuff too. Meanwhile Rust and Elixir showed you did not need the really up there stuff to have results in prod.
In the end what happened is that a lot of the highly abstract stuff was driven by "hype domain" that died, while more pragmatic but limited implementation burgeoned and absorbed some of them. Rubber met the road and that tampered a lot of people down.
There is still work being done, but rn it is more at the "experimental language" stage. Think Rust in the mid 00s.
Oh and Rust mindshare is still growing. A lot. A looooot.