One thing in your post caught my eye--you said that a Uhe plug-in had already been released as VST3. My original search did not yield much, but after you said that, I tried searching for "Linux VST3" and found this:
https://github.com/steinbergmedia/vst3sdk
It was here on this page that I saw that there is indeed Linux VST 3 support in the SDK (albeit a beta version). So it looks like Steinberg didn't give up on Linux as I had been led to believe previously!!
Yay!
So, does Linux Reaper currently support VST 3 plugins then?