>users can put in whatever tags they want. (Autocomplete is there to help, but they don't have to use it.) Then behind the scenes, human volunteers look up any new tags that no one else has used before and match them with any applicable existing tags, a process known as tag wrangling.

This is what booru sites have done for ages. Danbooru is the only booru mentioned in the article (and just as a passing mention despite being a few years older than Archive of our Own). I can only guess that AO3 is a bit more palatable than boorus for a general audience.

But this style of tagging really is the best of both worlds. The only downside is, as mentioned, the requirement for manual labor. I implement a similar system myself, obviously at a way smaller scale, for my own pictures. Basically I tag things as I feel like it at the time, and then every few months I "wrangle" the tags, as they do here.