Mine too, but with a British ISP that hasn't existed for fifteen years.

I'm able to log out and back into Steam using my plain username rather than email. The box actually says"enter account name" and wouldn't log me in if I tried to enter my email instead. I'm not sure if there are conditions to this e.g. my account was only created in 2009 and I'm not sure if I've ever toggled anything that "flipped it over" along the way either.

On the API side I know Steam has something like 3 different forms of auto generated alphanumerical account ID and I imagine that's what everything is really keyed off of on the back end.

I wouldn't swear it, but I think when I changed my Steam email the process didn't require access to the old email, just knowing the user and pass.