Since when? If push comes to shove and it still boots you can DFU restore the latest supported iOS version at the price of losing your JB. Samsung trips Knox and US models haven’t had a unlocked bootloader in like 5+ years, Moto voids warranty when you request a bootloader unlock code, Pixels just complains at splash screen if you try to unlock or load a 3rd party OS, but they don’t necessarily force you to load the latest OS unlike Apple.

Sideloading or rooting a phone does not void warranty. Although some (ie Lenovorola) makes the process of unlocking bootloader to allow a phone to load rooted or third party OS to go through their site that “voids” the warranty.