I still think you get OTA for it if on stock roms, just won't update using kies if I am not mistaken.
After tripping the counter you'll find that the check for updates under About doesn't work and gives you a message about your OS being modified. I confirmed that being rooted or not doesn't matter (unrooted, didn't fix it), using ART or not doesn't matter (flipped to ART then back, didn't fix it, also tried on another phone that never used ART, same thing), and modifying system files or not doesn't matter... it appears that once the flag is tripped you're done with *automatic* updates... what I don't know yet, since there hasn't been one to try, is whether we can still install an unmodified OTA file from recovery manually... it *may* just be the check that doesn't work but install still will, which would make this a non-problem in my mind. If it doesn't install though then yes, we'll have to look for someone to modify the OTA for us, or else we'll have to play the "wait for the tar, then flash image files individually and manually with ODIN from it" game (which may not work for the system partition depending on what you've done to it... at least on my previous Sprint S4, you *could* install an OTA if you were rooted so long as you hadn't changed anything else, and I *assume* the same will be true on a T-Mob S5).