I filter out HTML emails too, and a lot of mailing lists do as a standard practice. For some reason I find a large amount of sites which specify precise expectations for email are either German or somewhere in that region. Here's two examples:
The prevailing attitude seems to be "If you can't configure your email client to send plaintext or follow the other rules I've listed on my site, I'm not interested in communicating with you."
https://www.gaertner.de/~neitzel/mail-policy.html
https://www-user.tu-chemnitz.de/~heha/email.var
The prevailing attitude seems to be "If you can't configure your email client to send plaintext or follow the other rules I've listed on my site, I'm not interested in communicating with you."