Email address not available for Apple ID
I had an Apple ID associated with my Gmail account that I created a long time ago when I owned an iPhone 3gs. However, for reasons I can't remember, I created a new Apple ID using a different email and have been using it ever since. Recently, I attempted to delete my old Apple ID associated with Gmail as it was no longer in use, so I could use my Gmail in my current account.
After the waiting period for the account to be deleted, I've tried changing my current account to use my Gmail, but it says:
"Apple ID Error:
This email address is not available. Choose a different address."
Why is it not available if I've already deleted that old Apple ID account? What do I have to do to be able to use this Gmail in my current account?
iPhone 11 Pro, iOS 16