For at least the last twenty years S&W has pretty much said something like; Use commercially loaded ammo, not non-standard or reloaded ammo which has not been subjected to internal ballistic pressure testing.

Factory +P ammo has been tested. I use it in my 459 and 5904 but because they have an aluminum frame I don't want to fire only +P or +P+ out of it. I'm not going to find out how much it would take to damage it but so far it hasn't.

Honestly, if I had a mint 30 year old Model 59, I'd probably limit the +P. But, that's me.