My dealership said if they install a spacer or alter the suspension, a certified TOYOTA technician must install the parts at the dealership to keep the warranty.
That is a very specific violation of the M/M act in itself. They are forcing you to use their services... They cannot do that. It's not ok for them to say, "these parts are ONLY OK if WE install them" which indicates they are saying there is no other qualified person on the planet to do the work. A lot of places will try to pull stuff like this and YES a LOT of dealers will install a wide variety of aftermarket mods, even on brand new vehicles.
Still more send their vehicles out to local vendors like car stereo shops to install remote start units, u-haul places to get an aftermarket trailer hitch installed, window tint places, spray in bedliner places and even suspension places. None of those things are installed by "Certified TOYOTA Technicians" but they will warranty it as if it was, because it was installed as part of the sale. They already charged you double for it and they will have the original shop perform the warranty work anyway. My very best friend on earth is a manager at a high end car audio and aftermarket store in Clearwater Florida, been that way for over 30 years. He's on speed dial for any number of dealers and if they have a problem with his work, it goes back to him to fix it. It's just the way it works.
As for the original point, Let's say you install a 1/4" spacer to level the front end the day you bring it home. 5,000 miles later, you develop a leaky tail shaft seal or transmission pan gasket, or some other minor failure like that. There is NO WAY they can say "Oh man, we're sorry, you've voided your power train warranty sir"... As I also said, if you had a 6" lift with cutting/welding and lots of other backyard engineering done, they might make that argument stick a little better but it would still be absolute garbage.
At the end of the day, thank goodness we have what are arguably the most reliable vehicles on the planet. Worst case, you've done a small mod, you have a catastrophic failure, get the mod removed BEFORE you tow the truck to the dealer and you'll never have to worry.