there's a massive difference between tagging-for-self-recall and tagging-for-other-recall. when i invented tagging the first was paramount, but the latter has become dominant and has very different design considerations
one interesting note: you can infer a bunch of hierarchical information since people frequently tag from broader to more specific, topicwise.
some things can be tagged by multiple people and you can thus infer synonyms as well. this can thus be fixed in search.
Content tagging in online systems has existed since at least the 1970s, with the earliest example I can think of being MEDLINE https://en.wikipedia.org/wiki/MEDLINE
Right, MEDLINE's "tagging" system is MeSH, which is a large controlled vocabulary. MEDLINE does contain bibliographic data + journal keywords, but its real value add is MeSH, which is used for search, related publication identification, etc. in PubMed.
one interesting note: you can infer a bunch of hierarchical information since people frequently tag from broader to more specific, topicwise.
some things can be tagged by multiple people and you can thus infer synonyms as well. this can thus be fixed in search.