YouTube wasn’t always tied so strongly to a Google account, and overall fewer people had Google accounts in the first place.
I'm not aware of anything else that has persisted since then. Old email accounts, forums, games - everything is gone now and inaccessible for me.
They still have a page to recover a pre-Google account. It used to be a clearly outdated page with old graphics, now they made it a bit "better".
It's here: https://www.youtube.com/gaia_link