Where are Keywords, Favourites etc. stored?

Keywords and Favorites are stored in Libraries.

Keyword shortcuts (not to confuse with keyboard shortcuts - see below) are stored in the preferences.

Assignment of keyword shortcuts Control-1, ..., Control-0 is saved in the preferences:

I made two scripts a few years ago that allowed one to save those shortcuts and reload them.

I haven't tested them on the current version.

Resetting preferences does NOT delete custom keyboard shortcuts

They reside in

~/Library/Application Support/Final Cut Pro/Command Sets

Also: Workspaces live in

~/Library/Application Support/Final Cut Pro/Workspaces

With a little care, one may be able to just "transplant" the keywords from the old plist file to the new.

These are XML files.

One can open them in a text editor like BBEdit, search for FFKeywordGroups and copy the relevant part - beware that it is easy to this the wrong way and cause the file to be malformed.

I had these two keywords just for testing:

With FCP X closed, I was able to locate that section in the new preferences:

and pasted over the ones from the other file.

Started FCP X again and it worked nicely.

Apparently, yes.

One other thing: at least for me, when I delete preferences, I get a file on my desktop, like this:

VideoAppDiagnostics-FinalCut-2020-06-26-154529.tar.gz

It contains, among other things, a copy of the deleted preferences.

Now it seems that not everyone will get such a file. I can't say if I did something for this to happen, I certainly don't remember doing anything.