I finally had a chance to get back in my studio and look into this today. I found out:
1.) Cubase 10 has a note right in its plugin manager saying “Note: VST2 plugins are no longer supported.” There is a whole list of VST2 plugins on its Black List.
2.) My missing Eventide plugins – Black Hole, Ultra Reverb and the Harmonizer – are VST 2.4.
3.) Apparently VST 2.4 plugins will still instantiate in Cubase 10. The ones which did show on my plugin list – T Verb and Ultra Tap – are also VST 2.4.
4.) The ones that weren’t showing up were in some other folder where Cubase 10 wasn’t looking. I put them all in the folder where Cubase 10 was looking for them to be, and now I am able to open them in Cubase 10.
This is a good thing because Ultra Reverb is such a high quality work horse reverb, I use it on a lot of stuff I record. And when I record ambient sounds, for obvious reasons, Black Hole is all over every song I record.