I landed here looking for answers to these same concerns on my macOS Sequoia system running OBS. I dropped to a terminal and did a gren un the obs-studio directory/folder to look for where OBS code was still making reference to "ndi" and found some things in the obs-browser cache. See the following screenshot:
View attachment 116799
So, even though the plugin itself was removed, the OBS browser source was still depending on it. I had no Browser Source items configured in any scenes or profiles in OBS, though, so it was holding on to cache it didn't need.
What I did next was make sure OBS was closed, then removed both the "9761A96F922CBC77" subdirectory under the obs-browser directory, then removed those three specific files mentioned under Cache_Data. I honestly didn't care if I was going to break OBS. I'd saved my scenes and figured I would reinstall if needed.
When I reopened OBS, it still gave me the warning. I then changed up my grep to make "NDI" all caps and found a few more files to delete.
View attachment 116800
With reckless abandon, I repeated closing OBS, deleting the listed files, and attempting to reopen OBS again. But, alas, it's still giving me the warning.
So, it's not something in the OBS directories at all that's causing this on my macOS system, or at least not a text file that I can search with grep. I decided to take a look at my dotfiles (hidden files) in my home directory, and I did find a ".ndi" directory there (~/.ndi). I moved it to "ndi-backup" so it wouldn't be read as a dotfile/config file, and tried a third time to open OBS. Also unsuccessful.
I admit I'm at a loss now. No idea why the plugin browser is still showing a plugin that is not installed, not referenced in any source, and not even appearing in the OBS application's directory (at least not within a text file).
If I discover anything new, I'll return and post. Otherwise, assume I did some aggressive uninstalling and deleting of files to get this working again.