A couple of the commercial systems did this for sure. Linux definitely picked it up from UNIX practices, although I have no idea/memory of who did it first.  We used the idea at Stellar (Stellix) and at Masscomp (RTU). IIRC, a couple of others like Pyramid made have created a RAMFS - but it was kicking around the UNIX community for a fairly long time - certainly in the late 1970s - i.e. post V7.

FWIW: V7 had /stand which was a funky UNIX-like standalone system that some applications could be compiled.  The problem was that it was a little different so you would end up seeing #ifdef STAND in code for things like fsck, fsdb, even cat.  At Masscomp we ended up with three target environments for a couple of the system maintenance utilities: the OS, /stand and the boot ROMS.   This was expensive/a PITA to maintain and keep straight, and in the case of the boot ROM, space was a huge problem.

The RAMFS idea was created to get rid of at least /stand and IIRC we were able to drop a number of utilities out of the boot ROM.  I'm not sure how far they took it.   I left for Stellar and it was always the way Stellix booted.