Indeed they're not; they live on the 'user layer' rather than the 'application layer'. That's not to say many git-frontends (IntelliJ, Sourcetree, Github desktop) don't support them, but "git pullrequest" isn't a thing.

Edit: see "git request-pull" as mentioned below (file:///C:/Program%20Files/Git/mingw64/share/doc/git-doc/git-request-pull.html) but what it does is write "a pretty email" (the other poster's words) to STDOUT.