Some good examples have been posted prior to my reply here --- I'll reiterate Archive of our Own (fanfiction) and Danbooru (anime porn) as two fairly big sites with well-maintained tagging systems.

Both sites have abundant guides and documentation about their systems and it's very interesting to see how they manage the real-world complexity of their domains.

Here are some good entry points if you're interested:

Archive of Our Own:

https://archiveofourown.org/faq/tags?language_id=en

https://archiveofourown.org/wrangling_guidelines/2

Danbooru: (linked pages are text-only, but individual tag pages, as well as the rest of the site, can be highly NSFW)

https://danbooru.donmai.us/wiki_pages/howto%3Atag

https://danbooru.donmai.us/forum_topics?search%5Bcategory_id...