We definitely don’t wan to waste time on OpenARC. It has now been unmaintained for 7 years, as far as I can tell. (I wish folks would look at stuff like this before suggesting it. Unmaintained software is a terrible thing to add to a server on the Internet, especially in a role that interacts with unauthenticated incoming data from untrusted sources.)
The Fastmail implementation is seemingly maintained, though. But, it also does everything, so we’d probably want to refactor how we currently handle stuff like DKIM/DMARC/SPF, etc. in the interest of not having a half dozen different milters happening when only one can do all the jobs.
I’m ambivalent about the assertion above that a bunch of major mail providers are using it. I don’t think I’ve seen evidence of that. But, it is a real problem…if ARC solves it, that’d be nice.