Can't add old email to Apple ID even after deleting the Apple account associated with it previously

Apple does not have a statement about email address re-use in its documentation that I can find but here is an April 2021 post by ASC contributor IdrisSeabright that was recommended by the official Apple team -

"Once you've used an email address as an Apple ID, it can't be used again for an Apple ID, even if the first Apple ID has been deleted."

https://discussions.apple.com/thread/252629414?answerId=255095641022#255095641022

This is in accordance with what is claimed in this April 2021 post https://discussions.apple.com/thread/252629414

- Original post: "I tried contacting chat support and finally phone support today and they told me that you can NEVER use an email address that was used for a deleted account."

Khv1403 Said:

"Apple id: Can’t add old email to Apple id even after deleting the Apple account associated with it previously”

———-

Use a New eMail Address:

The eMail address has already been used. So, open a new eMail account, and then use that Mail address for your Apple id.

This seems to be answering a different question. Like the OP, I am not trying to create a new Apple ID with an email address that was previously used as an Apple ID. Instead I am trying to add the email address of a deleted Apple ID as an alternate address for an existing ID. For historical reasons I ended up with two Apple IDs, one with my main BTinternet email address as the ID, and the other (created later) with my mac.com address as the ID. I wanted to have only one ID, the mac.com one, so I deleted the ID with my BTinternet email address, and now I just want to add that email address as an alternate for my mac.com ID, but my attempts just generate unhelpful messages such as "The request cannot be processed at this time. Please try again later". Since it was my ID that I deleted, I can't see why Apple do not allow its email address, which is mine, to be used as an alternate for my remaining ID.