That's pretty cool. I wish Chromium supported this on Linux too. It seems more like a Chromium feature than an openbsd feature to me though? Linux programs installed via say flatpak have this on by default.
unveil(2) is an OpenBSD-specific feature, although you could accomplish something very similar with Linux and another sandboxing tool (or SELinux, but that might be overkill). I highly recommend you read the man page for unveil(2), it's very cool: https://man.openbsd.org/unveil
Yes, I am aware. I thought it was pretty obvious that when I said "it's a chrome feature" I didn't mean "unveil(2)" but being able to restrict access to the filesystem. Which is possible with both linux and openbsd, of course. Alas, the downvoters seem to disagree.
Yes, Ubuntu has it enabled by default - so Snaps are first class citizens on Ubuntu. I think the upcoming release of Debian may also have AppArmor by default.