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This is often repeated in these kinds of threads, but I don't think it's true. You just have to implement it in glibc, which can map the flags to the actual syscalls used on the current architecture and restrict the process to those with a seccomp BPF filter.

(Indeed, cosmo libc does exactly that, so it's definitely possible.)

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And then you run a program statically linked with a different version of glibc or musl and what happens?

Er... nothing? That's the point, if you statically link to glibc 2.51, then you get a version of pledge where "stdio" allows the stdio syscalls made by glibc 2.51 (say, "read" and "write"). Next time if you link to glibc 2.52, you get a pledge that allows stdio syscalls made by 2.52 (maybe "pread" and "pwrite"), etc. Nothing changes for the user because the libc wrappers for "read"/"write" are synchronized with the pledge promises.

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