As a native speaker, I can almost understand dummy value. But sanity check doesn't seem offensive at all. One definition of sanity is "reasonable and rational behavior."

so sanity check would be checking if the behavior is reasonable and rational.

I believe the thinking is they are insensitive to people with mental illnesses.

"Sanity check" hits home with me, so I guess I'll try to explain why it's problematic. I happen to use that phrase and variants on it throughout my code & comments as part of defensive programming.

I'm talking about having to put in gratuitous checks to make sure that the given data is plausible given the expectations of the business logic. My background is mainly in "legacy" applications where bad data can be a real concern. The intent is to make sure that the conditions are right to continue processing the data, with the alternative being either correcting the data or raising an error strong enough to get the bad data noticed.

The problem with "sanity" in this context has to do with social stigmas about mental illness. As someone that is mentally ill, I get it. Yup, my brain processes things incorrectly according to the world. Yup, this makes life difficult for me. But I am not "insane" despite my brain's broken data and twisted business logic.

While I am not personally offended or put off by the use of "sanity" in this way around software engineering, I agree with the general principle that judging "sanity" in code can be problematic. There are ways to express the same idea without implying judgement about mentally ill people.

"Sane" is one of my personal pet-peeve words. But only because people use it in an extremely approximated sense. It seems to always mean "good, for whatever meaning of good I personally choose to define, along whatever axes I choose to measure", without ever coming close to defining it any further.

Presumably they are examples of ableist discrimination against dumb people (those who cannot speak) and the mentally ill (insane).