To clarify: programming questions are not automatically off-topic here.
Instead, they are off-topic if they don't need "statistical expertise to understand or answer"; as we see under Programming in the help/on-topic:
if it needs statistical expertise to understand or answer, ask it here
I am not criticizing or challenging your response to programming questions but seek to clarify for any other readers what the actual situation is with regard to programming questions (because sometimes close-voters seem to miss this important exception).
[I haven't checked whether any of the questions you have responded to could actually be on topic under the above criteria -- I presume you're already well aware of this and have always been doing it correctly.]
I presume these people are terribly irate about this and serial downvote all my questions and answers, even across different SE networks
Typically they can only downvote in a very limited way before it gets detected as serial downvoting and is automatically reversed.
Unless they're determined enough to do it every day for many days (and know the approximate borderlines on voting behavior that the system checks for) it shouldn't make much difference to you, but if you think it's happening longer term (where it could start to bite into reputation in a more serious fashion), contact the mods and let us know, we can make some basic checks (though we don't see who voted on which questions; that's not a privilege available to mods).
If they're rude, flag the comment for mods.
I presume they're trying to get my account locked.
If that's their aim it won't work.
Do I keep out of it?
Comments are important for explaining the way things work to new users. You should continue to use them whenever you see fit to do so.
Do I just cast a close vote and move on?
When you think that's the best course, that's fine.
What have others done?
I comment a lot for a variety of reasons. I try to always be civil. Sometimes the responses are rude; if they're very rude a mod will remove them. Sometimes I get a few downvotes. Once you pass the highest rep level (as you have done already) what difference could a handful of downvotes make?