Is there an open source, cross platform file tagging system?

Are you suggesting to put all files in one folder and then fully rely on file search engines and AI to find anything?
I think this is maybe ok to do with something like notes and code, where you can do in text search to find stuff, but if its anything else then you need to fully remembering every file name or having a very good naming standard (which would then be comparable to file name based tagging, like Tagspaces, but still it less features and less consistency)

I wonder why nobody has yet tried to create an open source cross platform file tagging standard, preferably based on a central database (unlike Tag Spaces) to optimize performance

From my experience OCR is very unreliable and facial recognition probably even more.

This maybe works with notes where you can easily apply search based on the content of a note but not with data that is hard to systematically categorize by an automated system.

But even with notes its a lot more reliable to have a thing like a tag or a folder that you apply to all notes related to a certain topic and if you need this topic you just enter the tag and get all the stuff related to this.