At a conference years ago, someone proposed the idea of using this as the _only_ security mechanism in really tiny embedded systems. The implementation was 100-200 bytes in total.
The specific syscalls are quite new. Theo de Raadt gave this talk on pledge in 2015, and you can see that it was a bit different then. The "pledge path" feature never had a good enough API that didn't slow down the system too much, so they split that functionality into unveil a few years later.
https://www.youtube.com/watch?v=F_7S1eqKsFk