While I am sure that someone out there did use it (there's always someone!) this has to have been one of the niche-iest of niche products released by TSR, ever.
Raises hand quietly
Actually, I ran a couple of sessions with the Immortals rules back when I was in Middle School. I got it for Christmas and we created some "god" characters to see how it played.
It was, in fact, pretty insane. We switched back to the high level Expert/Companion level characters they were running and said "we'll play that later when we hit level 36" (Spoilers - we never hit level 36). The later "Wrath of the Immortals" boxed set rules were a much more sane version of the same idea (and much more playable - though again I only ever ran a one-shot with them to try them out rather than getting to do a full campaign).
And yes - our group was all BECMI all the time. A few of the players might have made their characters from the PHB, but I was running the game, and I didn't truck with all of that AD&D nonsense that felt unnecessary to the game, so we mangled their characters into a form that would work with BECMI with some house rules about separating races and classes instead of having "elf" and "dwarf" be classes and moved on. (It wasn't until much, much later that I realized that a lot of other people playing AD&D played a game that was very similar to the one that we were playing because they also didn't truck with a whole lot of AD&D nonsense and house ruled their way into the same kind of simpler game that we had house ruled our way up to. Kinda funny.)