EMACS/W3 originally supported SHORTTAG NET but was “fixed” to remove support. In practical terms, mainstream browsers couldn’t afford to parse SHORTTAG NET properly because it was very common to leave attribute values unquoted. You can leave some values unquoted, but not ones with slashes in. So the very common error <a href=http://xn--rvg would not get parsed as the author expected if SHORTTAG NET was enabled.

This is the earliest reference I could locate easily, from the www-html mailing list:

https://lists.w3.org/Archives/Public/www-html/2002Nov/0057.h...

You’ll be able to find more if you go trawling through USENET archives of places like comp.infosystems.www.authoring.html from 25–30 years ago, but it was a fairly niche subject even back then.

I think there were a couple of other niche tools that supported it, but I don’t remember the details after all this time.