That 200 lines doesn't include our pledge() implementation, which Make assumes is provided by the C library. Right now only Cosmopolitan Libc and OpenBSD have an unveil() implementation. It would take some thought to decide what the best approach would be for incorporating something like that into GNU Make. So I'm waiting on more feedback from the community and the GNU developers. Because upstreaming is totally something I'm open to considering.

Also please consider that GNU Make supports so many ancient platforms, like DOS, QDOS, Amiga, Windows 3.1. When I forked GNU Make, the first thing I did was delete all that support for ancient defunct platforms to give me enough room to think about how to approach implementing this feature. I don't think I could have done this if I had to wade through all that code from the start. I'm doing this work just for fun and to help out, and coding is only fun when the code is clean.