The kernel can easily version APIs using struct size, as long as new members are only appended. The libc function would pass the size of the struct to the syscall, or you could have pledge() itself be a macro that computes sizeof in the caller.
That is the exact solution described in the link I included in my comment (I am the "Aleksa" in that article). It is not entirely trivial (certain edge cases need to be handled) but it is entirely doable. But string arguments also work if you don't have complicated data parsing requirements.
I (obviously) prefer the extensible struct solution but there are downsides (and other solutions weren't an option for Linux anyway).
