New Ardour user here, I was installing vst plugins (from vst4free).
Unfortunately some of them were only .vst3 files and Ardour seems to scan only for .dll files.
Since, I don’t know how the internals of Ardour and I didn’t find anything on the subject I had to ask.
You should be able to do that for VST3 in Ardour 6.6 (there was a bug pertaining to that in the 6.5 release).
Yet, one of the great things about VST3 is that there is a dedicated folder for plugins that all audio applications can default to. One fewer configuration step, and things can just-work.
Compare that to Apple macOS/X: the path for all plugin standards has always fixed. Things just worked and most users don’t even know or care where they are.