Disassociating a closed Comcast email account from iCloud on iPhone

Is the account just added on the iPhone for email in the mail app then? It's not actually associated with iCloud, just available inside the Mail app?

If so, you can just remove it in Settings ➜ Apps ➜ Mail ➜ Mail Accounts ➜ The Comcast account ➜ Delete Account.

If its added a secondary email address on your Apple account in addition to the mail app or separate from the App entirely then you can remove it from Settings ➜ Account (top of settings) ➜ Sign in & Security ➜ Tap the Comcast Account, and then on Remove from Account.

In this case though, it should not be asking for a password at all, as its not linked to it in any way, it just added a secondary email address to receive notifications and other emails from Apple, so does not verify its existence to require a password.

Check your accounts at Settings > Apps > Mail > Accounts.

Each account has an Incoming Mail server and an Outgoing Mail Server (SMTP) where that old Comcast email address may be entered. Either of these email addresses can be different than the actual email address of your account. You can either edit the email address for the Incoming and Outgoing Mail server or if that email account is not longer used, it can be deleted.