I call hogwash.
the amount of resistance an engine puts on the driveline during engine braking is paltry compared to what it can do under load. The max psi /against/ the cylinders will be 1 atm (15psi), as a vacuum, whereas under power is somewhere between 600 to 2000 psi. That directly correlates to driveline force at the TC and everywhere else.
the only moment I cringe is if you implement a poorly timed downshift where the clutches have to raise the engine rpm from no-load cruise (say 1000) to braking (say 3000 or more). If I’m manually forcing the downshift, I might blip the throttle to rev-match.
TC fluid shearing and heat load I think would be very inconsequential. The greater risk I would consider is unloaded piston forces at upper rpms at the tops of their travel.
You mentioned rev-matching. Were you talking about manual transmission or auto?
Sometimes when I'm in 5th with TC locked, I give it a little gas and TC unlocks and rpm goes up a little and then I downshift to 4th. I'm sure with the small delta rpm (from 5th to 4th) this is not necessary or may not even be advisable. I do it more often when down shifting from 4th and Locked to 3rd. I used to drive manual trans forever but had no choice with the new truck and maybe old habits.
Anyway, curious how you do your "blip the throttle to rev-match." since you can't go to neutral and then downshift like you can with manual trans.