Doubtful. Even among the Reiser users, I bet nobody uses a ReiserFS version bundled with the kernel. Reiser4 is has never been upstreamed which is a bit of a shame because technologically it's apparently quite good. Nobody but the leftover developers know why they did not just rename that thing and adapted it to kernel standards...
I was one of the reiserfs users before it happened and I dumped it quickly. I think an awful lot of people did the same, and since it was one of a number of alternatives to ext I think it is more abandonment than obsolescence.
Now I think my decision was wrong headed, so long as he was no longer involved. The NASA example was given. Alfred Hitchcock movies? Abuser of women but damn good films.
reiser4 was a promising filesystem (had transparent compression very early on) but reiser3 (reiserfs) didn't really have anything significant over ext3/4 so abandoning it made sense. And reiser4 never made the kernel so it's understandable that most never bothered with it, although I used it for a few years before btrfs became viable.
At the time the competition was against a nascent ext3, as I recall, against which reiserfs had significant advantages. Journaling wasn't standard back then and wasn't handled by ext2, so there was a lot of competition, and it wasn't clear that ext3 would be the best solution.
Now that I think about it ext3 wasn't even a thing. And when it did come on line reiserfs already had a mature journaling system.