I always tell Claude to turn that off. It looks incredibly tacky. I'll leave an agents.md file in the repo if I don't care about people knowing AI helped me.

Waiting for the day I get an email from someone that says "co-authored by my iphone"

Yes, but the equivalent of "sent from Invasive·marketing·of·some·MUA™" would be "generated with Ludicrious·Laughable·Mockery™".

That is, the MUAs don’t add "co-authored by myMUA".