Regarding SRFI 227 (Optional Arguments):

We have "vendored" a few dependencies into the srfi-tools source tree so
that users wouldn't have to retrieve them from package managers. (As you
experienced first hand, retrieving a set of dependencies across a range
of Scheme implementations can be a burden.)

IIRC, SRFI 227 can be implemented using syntax-rules so it's portable in
principle. But the implementation is a substantial amount of code and we
only make trivial use of it. IMHO it would be best if we could get by
with a subset of the SRFI only.