Not illegal, from what I've heard — someone read the small print of their mortgage agreement, modified it, signed it, the bank person signed the modified agreement, the bank sent junk mail, he pointed out this meant the bank was in breach of contract and given how he'd modified it that he was apparently theoretically entitled to write off the remaining debt.

Apparently that last bit would probably not have survived an actual legal fight, but they never sent him any more junk mail.

(This was pre-GDPR, so perhaps things have changed?)