I have not checked with OpenBSD 7.9, but as of 7.8 it did not support --exclude or -z. But outside of that openrsync works great.
(EDIT: --exclude is now supported on 7.9. Not sure when that was added, nice!)
But seems avoiding "slop" is getting very hard. I saw postfix now has a bit of AI code in it.
Where do you see that about Postfix? I followed the links and the only thing I see is that AI is being used to find bugs, not write code.
Exclude is very commonly used in automation jobs to avoid duplicating big git repos and other big files. I think that would be a show stopper for a number of people.
Somewhat ironic Postfix has a record of no root/RCE in the default install, where opensmptd hasn't (CVE-2020-7247). Time will tell if it stays that way.