One of the original author of Ribbit here, glad to see it is still getting attention. Léonard and Marc took the project much further than I imagined.
As a fun fact, the project started as a riff of Justine Tunney's famous https://justine.lol/sectorlisp/.
For the interested, my new favorite project from Marc's team is https://github.com/udem-dlteam/pnut
One of our goals was to reduce as much as possible a R4RS compliant REPL, because it is quite reasonably sized and doesn't support macros (we managed to fit it in 6.5KB!). We considered R7RS, but it was a bit too ambitious for a first goal. We plan on supporting R7RS eventually !
If you want to read more, section 4 of "A R4RS Compliant REPL in 7 KB" explains our rationale behind the choice of R4RS :
I'm pretty sure we had support for JS, C, Python from the beginning as well. Then Scheme, posix shell, assembly came along for the ride.
> I'm unsure how support was kept throughout the versions but I know a much larger set of languages now support the runtime, at least for a subset of the features
One thing that really helped is CI testing! All the RVMs (the 16 targets) are now fully tested at each PR (https://github.com/udem-dlteam/ribbit/actions)