Apple ID
When linking new mail to an Apple ID
Writes this email is unavailable select another one. But when you try to log in using this almost Apple ID website, it says it's not registered
iPhone 12, iOS 15
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"If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it." From: https://support.apple.com/HT202667
Apart from the above statement I have yet to find a formal Apple position statement online about re-using email addresses. Many user reports on this forum (of course they could all be just making up things but I don't have specific reason not to believe hundreds of independent topics over many years) suggest that contrary to what the statement implies as to ease of address re-use, if you have recently used an email address with Apple for another purpose such as a notification email address it cannot be re-used; at least not immediately and possibly never. I guess you can "try to use it" but many users have claimed this does not work.
About your Apple ID email addresses (was "Manage your Apple ID primary, rescue, alternate, and notification email addresses") --> https://support.apple.com/HT201356