It looks like Apple make their fork of openssh available here: https://opensource.apple.com/source/OpenSSH/OpenSSH-236.100.2/openssh/.
It includes native system keychain support which is pretty handy. Not sure if the preference would be a separate package or not.
USE-flag ? (if it builds at all?)
I would not be opposed to a USE flag, but ideally the minimal code changed to implement this should be separated out in to a patch. A link to an online repo (is it git? is there a clone URL?) isn't very useful.
If you attach a .patch file I can take a look at it, and see how much effort it would be to maintain with future OpenSSH releases.
Sam, Apple isn't known for creating clean patches. In this case we really need to know a) if it can be built, and b) what it would entail to make this a patch, and c) if their code is compatible with a clean upstream release.
My sense is that if it builds, you probably want to keep it in an overlay, as it will benefit a very small portion of users.
Homebrew ran into this as well: https://archive.is/hSB6d#10%25
You're both right, of course. I had a look at this and the dumps on the site aren't particularly new (latest is ~Sept 2019).
What *is* promising is that MacPorts seem to have a patchset they use.
I'll assign this to myself for now to make clear I'm not expecting anyone to act on this - just as a reminder to look into it more.