Cant change email, because domain doesnt exist anymore

Hello,
I have this account for a long time already, unfortunately back when I was creating this account I had an email on [email protected] (yes I was a child back then…) unfortunately, their email services were discontinued back in like 2018 or something. Meaning I had to transfer my email to Gmail. Ofc I forgot to do that on this account. As You can see I still have access to this Account. I don’t know how I can prove that this is truly my account but I can try to show old archived emails on my Gmail account, my origin account or Steam account where I have a moderator on VU steam community.

Greetings
Killer200120, John