It's 50GB now. I've been digging into how they do it as I'm a casual user of a few hundred GB/mo on both T-Mobile and Sprint.
From what I can see and from the Sprint CTO's comments, the deprioritization happens on radio layer working on slices of around 30ms. If you are near cell edge, the scheduler will assign you less physical resource blocks. This deprioritization will affect you much less if you are near the tower as higher orders of modulation and less error correction plays into it. (Think out of a possible 100 or so blocks you could be assigned in a given timeframe, the blocks can either be slow due to poor radio conditions or fast)
From my year or so of having a dedicated testing phone on T-Mobile, I can feel confident in saying what they say is true about speeds only being affected on congested sectors (not the full tower or region). Sprint I haven't had as much time with so I can only go with what they say is the case.
Ah, Verizon is still 22GB for the "new verizon unlimited plan" and "Beyond unlimited". "Above unlimited" is 75GB, and "Go unlimited" deprioritization is possible from the start.
I'm still on the "new verizon unlimited plan" which is the same that "beyond unlimited" provides, just at a cheaper price.