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.
