Are you talking about Ovm https://dl.acm.org/doi/10.1145/1324969.1324974 https://apps.dtic.mil/sti/citations/ADA456895? pizlonator (the Fil-C author) was one of Ovm's authors 17 years ago. I don't think it's in current use, but hopefully he'll correct me if I'm wrong. The RTSJ didn't require a real-time GC (and IIRC at the time it wasn't known how to write a hard-real-time GC without truly enormous overheads) and it didn't have a real-time GC at the time. Perhaps one has been added since then.

I don't agree that "it is a niche of a niche". There are probably 32× as many computers in your house running hard-real-time software as computers that aren't. Even Linux used to disable interrupts during IDE disk accesses!