There is a kinda “solution” for this. The following costs a little but its solves a lot for me. Mind that I am on a MAC (but it will work in most PC’s as well) and that I have VST, VST3 and AU versions of plug-ins and instruments installed. If one of them do not work, the other will, at least for those tested.
You need to spend EUR 99 on Blue Cats patchwork. That thing solved lot for me. I had a bunch of instruments and plugs that did not work after upgrading to Cubase 9 (and even some trouble in Cubase 8.5). Patchwork acts as a wrapper and can host a lot of software that Cubase cannot.
So far I have tested the following that does not work in Cubase 9 on my computer:
Addictive Drums - XLN
Chroma Key - Audiothing
Anuna - Crysonic
Sindo - Cysonic
Izotope Ozone 5 & 6
Izotope Nectar 2
Izotope Alloy 2
Ultra Channel - Eventide
Tokyo DAwn - Feedback Compressor
Tone Boosters _ Many mix and master plug-ins
Cable Guys . Shaperbox bundle
It also handles my old Lexicon MX 200 software for the external hardware unit.
The same thing lets you load all VST in Logic if that is desired and the other way around. Nowadays most of instruments and plugs are both platforms so I do not seed the real value of it for that reason, but as mentioned when one platform is not working the other will. PC users will not have that option but at least be able to try out VST or VST3. Patchwork also comes with it’s own bunch of FX units and a lot of presets. You may put units in parallel or serial.
You can also buy Patchwork bundled with 32 bit lives that will turn most 32 bit plugs into a 64 bit interface. This lets you use most Powercore plugs and instruments up to Sierra OS!
I do not own Patchwrok myself but I have downloaded the demo and done my testing there. I will for sure buy it as soon as money hits my account!
I am in no way affiliated with Blue Cat nor do I have any advantages from “marketing” patchwork. I have faced thaw same problem as the OP’s.