If he's retiring, he's LTIRetiring. Whoever claimed earlier in the thread that you can't go on LTIR if you have career-ending surgery doesn't know what they're talking about, and I can't find any source anywhere for the Leafs supposedly threatening to not pay Horton if he got career-ending surgery (and there's absolutely no chance that would ever fly with the NHLPA). LTIR exists for the express purpose of teams dealing with long-term injuries. Long Term Injured Reserve.
He'll get every penny of his contract even if never plays another game. He's absolutely 100% injured if you read the Athletic story about him from a few weeks back. There's no reason whatsoever for him to retire for real. All he has to do to collect $21m is report to camp and take (and fail) a physical every year.