It is somewhat rare but not entirely unheard of for carriers to have agreements with the major streaming providers, and so in some cases the streaming provider

It is somewhat rare but not entirely unheard of for carriers to have agreements with the major streaming providers, and so in some cases the streaming provider will actually aid in the process. Most of the time, however, this isn't needed because what's actually happening is limiting bandwidth, not resolution, but there is a very strong correlation between the two, so by limiting bandwidth you end up with the desired effect of limiting resolution. These days most streaming providers use some form of adaptive streaming in which client-side logic decides to get bigger or smaller "chunks" of video based on how quickly prior chunks downloaded. A rudimentary solution for a carrier would be to simply implement logic like, "if throughput to device X > someLimit, add a delay in delivering packets to device X". From the client's perspective, getting the bigger (and higher quality) video chunks will take too long, so it will naturally shift to the smaller (and lower quality) chunks.