Does fspy add-on work in blender 2.81? I have tried installing fspy in blender 2.81, however it says it is 'installed' (as can be seen in attached image) but then doesn't appear in add-ons list, and hence cannot be enabled.
You installed the wrong file; you need to install fSpy-Blender-1.0.3.zip
 (or newer) from the Releases page.

The file you're using, fspy-Blender-master.zip is likely the source code and doesn't work as addon.

It's not very clearly stated on the page and it's easy to download the wrong file(I had the same issue :)