They can write to you forever if they wish however If the last payment made to the mortgage account was sometime in 1989 and no further payments (no matter how small) have been made since by you or your ex then the lender is now 'time statute barred' and will fail in any court action they take against you if this is pointed out in your defence papers.(posted 7343 days ago)This is due to the Limitation Act which under section 20 (mortgages) gives a claimant (lender) 12 years from the last cause of action (usually 3 months after the last payment) to take legal action.
It sounds to me like this is a feeble attempt to obtain some money from you. Incidentally, once the claimants rights of limitation have expired they cannot be reset by making a payment.
If you continue to receive letters from them to this day which threaten court action respond to them with this fact. If you are unsure if your ex has made any payment to them then point out to the lender that you require proof of the limitation period being reset.
Check the many previous similar postings to your own on this site for more detailed information