Horrible enough, in the actual Nazi "euthanasia" program, institutionalized mentally or physically disabled, and psychically ill people were murdered. This was done in as much secrecy as possible, without any published laws of the kind, and the families were given fabricated natural causes of death after the fact.
What we see in this series, is a thinkable, but fictional extension of that idea. Historically, there was no law or program that required family or physicians to turn over patients to be killed.
And in answer to your question, there are no sources that indicate any plans killing people for the mere fact that they were sterile or elderly.
Given the unwanted waves that the "T4" action made already, I really doubt that that either would have ever happened, even after another 20 years of propaganda. The majority could be manipulated as long as the consequences concerned ever only "them", not "us".
The show has alluded to killing the elderly, to an extent, as long as they could function in some capacity. But being unable to have kids, for women in this show, is most likely a death sentence. Remember Lucy, one of Helen's friends. She couldn't get pregnant for 3 years and she was VERY worried about the consequences of that, and you could tell that it was likely ending up with death.
Oberhruppenfeuhrer Smith has to kill his son because the boy has a degenerative disease and is considered "defective." So useless eaters were not just elderly or mentally ill, at least in this show.