they only exist on OpenBSD. They're used a lot for default OpenBSD binaries, but they are mostly hopeless for openbsd developers/porters to try to apply to portable software after the fact if the upstream devs aren't supporting that work.
(There are other mechanisms like containers or freebsd' jail that try to accomplish the same thing, but those tend to be "lots of functionality inside the sandbox" solutions, whereas openbsd is mostly aiming for "allow nothing besides the minimum in the sandbox".
(edited for clarity, thanks for the nudge ghoward).
I was referring to the OpenBSD devs trying to hack pledges into other people's software by trying to guess what capabilities it needs while porting. That's not a good thing to try to do -- see also all the various attempts at guessing seccomp profiles in the Linux world, which has the same problem.
Has been done to some 3rd party applications. Notably Chromium is pledged an unveiled, Firefox (unveil), pdf readers, compression tools, mail clients, ...
(There are other mechanisms like containers or freebsd' jail that try to accomplish the same thing, but those tend to be "lots of functionality inside the sandbox" solutions, whereas openbsd is mostly aiming for "allow nothing besides the minimum in the sandbox".
(edited for clarity, thanks for the nudge ghoward).