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.
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.
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.
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.