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 can't answer as to why the current version is R4RS specifically. The original version Marc and I wrote was running a subset of R4RS. We tried to minimize the code as much as possible (hitting the 4K mark), so it's very possible we just wanted to get everything out of the way (it has been a while)
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 :
https://arxiv.org/pdf/2310.13589