I'm pretty sure that is just a rumor. TYM is still making the 15 and 20 series tractors. They discontinued one HP configuration. (For those unaware, the 2515, 3015, 3515, 4218 and 4815 are the same except engine/engine tuning. My understanding is they discontinued the 4215. I would expect maybe they would delete the 3015, too. Not enough difference to make a separate model.
I got a T574 vs the 5520 because it has some convenient features standard that weren't on the 20 series.
My guess is that when/if TYM phases out those tractors, they will blend the features of them. The 474, 4815, 4820, and 494 all use the same engine. The 5520 and 574 use the turbo version of that engine.