Couldn’t you just look at a list of tags?
The number of tags should be more or less similar to the number of non-leaf nodes in a hierarchy, or else you aren't capturing the same information. Any tag that applies to more than half of the files is probably useless. On my blog, the tags "blog" and "technology" are definitely useless. That's fewer than 250 entries and already it has cruft.
Were people consistent when they added tags? Does your system suggest tags automatically? Is this actually a full-text search minus stop words? Is there a librarian who cleans up after you and merges tags that have the same meanings? Would the full-text search be more useful than tags?
I use ... a lot of tags.
(I couldn't tell you without downloading my entire archive and doing some magick to count them, as Pocket won't give me a count itself.)
Pocket lags by well over a minute in even starting to populate my tags list after I enter some text into the field. (Chrome/Android, using an external keyboard.) It similarly takes several minutes to scroll through the tag list using either the Web client or the Android app.
You could argue that this is a piss-poor UI/UX issue, and I'd agree. It's piss-poor UI/UX issue that Pocket have failed to address since I first reported it over five years ago.
https://old.reddit.com/r/dredmorbius/comments/5x2sfx/pocket_...
If you'd like another list that has rapid-scannability issues, there's the Library of Congress Subject Headings:
These are publicly available. In 27 PDF files, corresponding to printed books ranging from 12 to 842 pages in length: