If you used Hide My Email, then it will be located in Settings > [your name] > iCloud > Hide My Email
Is the account you are looking for listed there?
If you later went in there and deactivated that item, then it's gone.. you will not get emails forwarded from that address.
There are a few different ways of utilizing this type of thing..
If you created an account and used Sign In with Apple, you could also choose to hide your email.
If that's what you did, then it should be listed in Hide My Email and it should say Sign In with Apple
I'm not sure how you would undo Sign In with Apple
If you created an account on a website or an app, chose Hide My Email, but entered your own password or did not save the password.. then it should still be listed in iCloud > Hide My Email. And if it's listed, I would think you'd get emails forwarded.
I am not sure what you'd have to do if you are not receiving the forwarded email. Check your spam/junk folders?
If you created an account on a website or an app, chose Hide My Email and then also let the iPhone suggest a password or you saved the password to your phone/Keychain, you should be able to find those credentials in Settings > Passwords