Vabungula
Vabungula is a personal constructed language created by Bill Price in 1965. He may claim his language to be rather uninteresting and that "there are other constructed languages out there that are more clever and glamorous", but the sheer size of its vocabulary, the amount of lengthy texts translated into it, and his overall immense level of dedication to the language lasting over 50 years (including his alleged ability to speak it fluently) makes it one of the most fascinating constructed languages out there in my eye.
Unfortunately, the website which was hosting all of Vabungula's extensive documentation went offline sometime in 2019 or 2020. There does exist an archive of the Geocities website that used to be the home of Vabungula, but naturally it is over 20 years out of date. I don't expect that much of the grammar has changed since then, but the dictionary is certainly missing a few thousand words that had accumulated in the two decades following.
The Internet Archive is the obvious solution to things like this, but I haven't had much luck with that since I don't remember the website's domain nor have I been able to find it anywhere (it seems people only linked to this language back when Geocities was a thing...). However, the website was apparently moved to an iWeb domain during the fall of Geocities, and the Wayback Machine does happen to have an archive of that homepage from 2008, although seemingly none of its other pages.
Luckily, I happened to archive the entire Vabungula dictionary in plaintext form in June 2019, shortly before the website went down. I can't guarantee that it's completely up to date, but it should at least be very close.