The T4 programme started in 1940. There were public protests of the T4 programme in 1941. The T4 programme was the mass killing of disabled - mostly learning disabled- people.
The T4 killings are seen as the precursor to the mass extermination of Jews, with refinements of killing methods.
Six different camps were used. People were killed in gas chambers, or by lethal injection, or by starvation.
That's all true, but we're talking specifically about the extermination of Jews. Is there evidence that the Nazis planned T4 as a trial run of Jewish extermination, and if so would NYT have known about those intentions in 1939?
The Dachau concentration camp you mentioned has a gas chamber but it wasn't used if I remember it correctly (was there on a school trip).