OpenBSD now has some support for creating a runtime dir during login - it's not always going to be available but platformdirs should probably align to the path used for this, /tmp/run/user/$uid, as if available ownership and permissions should be set correctly.

https://github.com/openbsd/src/blob/dca435757bf3e16793666be5cd412e56d227894c/lib/libc/gen/login_cap.c#L581

(/var/run/user/$uid as currently used by platformdirs is very unlikely to be writable by the user).