So it turns out BitKeeper is still a thing (yes, Linux greyhairs, _that_ BitKeeper), and Open Slopware lists it as an alternative VCS:
https://codeberg.org/ethical-foss/open-slopware#user-content-version-control

Even more amazingly, it's still being updated. However, I regret to inform that its lead dev was slop-shotted, its only side branch was slopified last Friday, and PR#15 is open to pull it into main:
https://github.com/bitkeeper-scm/bitkeeper/commit/0b292334aa83fdb434d84f3e8715a0a43e1fdac1

Some pre-slop forks already exist:
https://github.com/bitkeeper-scm/bitkeeper/forks?include=active&page=1&period=5y&sort_by=last_updated