I am waiting for an answer from one of them. Being an "isolated" user, I'm not going to ask them all.
For now, I got the following feedback on a French forum from a reliable user: FCP 10.5 and Big Sur on Intel:
CoreMelt: it works (I have 4 of them)
My FCPeffects: ok
Neat video : ok
iZotope RX7 : ok
Hawaiki (fxFactory): ok (Yanobox from fxFactory also works, I tried it)
Pixel Film Studio: no but I didn't check if an update was planned,
Hitfilm: no, and I'm sorry because I had their range for free and I don't think that's possible anymore...
MotionVfX: no, there will surely be some Updates.
I moved the testing machine (Mac mini) over this weekend, and am having instability and beachball with Coremelt packages. I would say it's not ready for a production switch yet although the plugins do load and seem to preview correctly.
Pretty vanilla system - Mac mini 2018 with clean install of Big Sur and FCP 10.5 + the latest Coremelt package linked above (512.13).
Stabilization works for me but Chromatic and Gadgets don't. Haven't tested exhaustively on everything yet.
For me also it's a question for updating a personal and test machine.
Shame on me, for Yanobox distributed by FxFactory the talented creator write me that the information is written at the bottom of each plug-in's web page... ... compatible with 10.5 or less on Mojave, Catalina, Big Sur and often High Sierra. For information, good news, Nodes 1 and Nodes 2 versions stay compatible with Big sur.