imagine people would start defeding email

There are literally globally maintained blacklists of spam email sources. When people lease a static IP address the first thing to do is to check it against the major email blacklists.

It happens to email ALL THE TIME, we just call it something different when it happens to email. Evaluating email for SPAM potential is an every-day common place occurrence, and for at least the past 10 years, a factor called 'domain reputation' is part of the equation. Entire domains get spam blacklisted because they refuse to enforce rules for their users. The end result is that some domains completely refuse to accept mail from some other domains.

Blacklisting an entire domain can and does happen daily. It just doesn't have the same triggering ring as the word "defederation" has.