It’s the second field on the edit profile page. Can’t recommend putting it in, but victim blaming doesn’t help anyone that already did so.
The edit profile page has a statement that “providing your real name can help friends find you on the Steam Community” with no indication that doing so also puts you at the risk of capital-G Gamers. I can see quite a bunch of people thinking that that’s perfectly reasonable and not going to be abused.
Given that the UUID changed, you almost certainly made a new LUKS container, overwriting the old one. That’s bad, because the LUKS header is the only source of the actual encryption key that was used, and making a new one will overwrite both the main header as well as its backup copy immediately. Your password/keyfile/whatever is merely used to decrypt the part of the header that has the actual encryption key, and that’s gone in that case.
Unless you have access to a header backup from before that, there’s a fairly strong chance it’s irrecoverable. I’d suggest going through any archives you might have to see if you have such a backup - most of the instructions on the Gentoo wiki encourage making one, so you might have made one through the power of copying & pasting instructions. Should be a file of around 16MB.