OpenBSD already prevents syscalls from writable pages. It did not break JITs, in fact, to quote Theo de Raadt, it brings over an aspect of W^X to JITs that haven't been adapted to work with mandatory W^X.

https://marc.info/?l=openbsd-cvs&m=155942895309114&w=2

This means it's forced to use the libc stubs for syscalls. Any direct syscall from a JIT is suspect.