The following is my take on the issue. I am not aware if this is how BT justify it.
Prior to October 2022 a BT Broadband account holder could create eleven email addresses and either keep them all for his/her own use or "gift" any number of them to friends or family.
In the past, when the BT Broadband account holder left BT all the email accounts would be closed regardless of whether or not they were "gifted" unless the BT Broadband account holder took out a BT Premium email account. This would include all eleven email accounts and thus keep them all active.
BT do not know which of these email accounts are the broadband account holder's own email accounts and which are "gifted" to others.
If BT was just going on the fact that all of the email addresses were the property of the ex broadband account holder, that would mean that if any person with one of the "gifted" email accounts took out a BT Broadband package, all the email accounts from the the original BT Broadband account holder would be transferred to that new broadband account and the original owner would lose ownership of all eleven email addresses to the new BT Broadband account owned by the "gifted" account holder.
Hence the reason that the email account holder must move them to a current BT Broadband package and note it does not necessarily have to be one that they are the BT Broadband account holder of and if they have a BT Premium email account they must cancel it.
Now when a person leaves BT Broadband, all the email accounts are "downgraded" to BTMail Basic which is a free service albeit more limited than the standard email account. To get the same as the standard email account that they had they would need to take out a BT Premium email account.
