Filesystem-oriented flags: sad, messy and not going away

The solaris model is that the syscall API is a shared object (along with the rest of libc). User-space code doesn't get to make syscalls at all. The libc shared library can be versioned and indeed multiple, so you can (theoretically) keep older cruftier ones only as long as you have any executables that need them, on an install-by-install basis. I believe that OpenBSD is considering a similar scheme in order to have some sort of protection about where syscalls can come from, to prevent trampoline and gadget-style malware, perhaps.

The BSD versioned syscalls are in the kernel (so you can still have static executables), but they can be supplied by loadable kernel modules (as the linux and SCO syscalls are/were), which can eventually be deprecated or not loaded as suits the use-case, without getting (too much) in the way of the "fresh" syscall API.