I think the term itself only appeared in 2000's but it started much more earlier and studies about its prevalence in Japan refers to issues from as early as 60s.

Well, if you ask me, Hikikomori has been a thing in Japan since ancient time lol ( Amaterasu locked herself in a cave following an argument with Susanoo. And, other gods kept being festival around the cave to get her out lol...well, that's the short version of it.)