I'm sure the team are having a rough time with all this, but I don't understand the path you followed to go from the maintainers have not taken this action until now to "This doesn't speak well to the security headspace of the Arch maintainers". Or what "security headspace" means exactly.

Yes, lots of people thought disabling package adoption is/was a good idea. It seems quite likely that the Arch DevOps team also could have come up with that one, and they certainly wouldn't have missed all of the people telling them to do so.

It seems to me that disabling package adoption is not a desirable thing to do in general, and can only be used as a stopgap response to an emergency, which seems to be what is happening right now.

"just disable adoption" certainly can't be a long-term solution: Without adoption, the AUR will slowly fade away as orphaning a package would be permanent. Maybe the idea is to have some sort of approval process to filter adoption requests? In that case the AUR just dies instantly as that would effectively be another official repo with all of the issues that that would bring.