If someone asks for a TL;DR, I wouldn't mind. At least, they are interested in what you are saying. Providing a TL;DR also helps language learners.
Of course, it depends on the general attitude.
Sharing a TL;DR summary is different from simply declaring it.
“Too long, didn’t read” as a stand-alone sentence is an asshole move and only serves to detract from motivation to share theories and ideas.
I try to assume people online have good intentions until they give me a reason not to. "TL;DR?" conforms to netiquette.
"You lost me at" does not, and is indeed rude af.
If you post a long theory and do not include a TL;DR, then it is normal that people come forward to request it. Do that in a mailing list, and people will chew you up. On a message board, it is optional but still very helpful.
Personally, I assume periods for everything unless in caps or it has one of the “what”, “how”, “where”, etc. They need to end it with a question mark or exclamation otherwise.
Regardless, in the instances I think of, even assuming the nicest punctuation possible, it’s no dice with what they include with the TL;DR.
And assuming a question mark is looking at the world through rose-tinted lenses, because I have yet to ask for a clarification, and not receive a clarification that they were being dismissive.