3rd party plugins gone in Final Cut Pro

The effects browser is set to show only HDR effects (see the letters HDR in blue near the search box. Clear that and see if they show.

Which plugins? I am thinking that by plugins you actually mean templates (titles, transitions, effects and generators). Can you post screenshots in Column View showing exactly where they are?

And what exactly do you see in FCP where they were supposed to be?

Third party plugins are not affected by updating the OS or FCP - unless they were improperly installed inside the application bundle - which apparently some developers wrongly advise people to do.

Are you still using the same User account?

Never buy from PixelFilmStudios again. This is because they are a group that steals the skills of developers working for FCP users. There's a reason it's cheap. I want good users to make the right choices.