Funny thing is, i’ve never heard about plaintext/markdown mails being enforced over the usual html-with-potentially-scripts-and-hidden-URLs.

This is always the part that drives me up the wall. Literally the default behavior in Thunderbird because it's built by people that care about privacy and security before anything else. So many features to make email "prettier" and "easier" except all they do is introduce new ways for bad actors to hide their actions from attentive users