Kmail is already capable of creating html emails as well as displaying
html emails.
The default is simply off.  It can be activated in setting for full on
use or as needed per email.

I agree, html email is bad and its being abandoned by the majority for
security sake.
However Tim is correct, the option should be there.,

The most logical solution is to repeat what was done on 3.5.10, make
it available with off as the default.

Off to sleep!

Kate