I have seen the use of
tl:dr; a fair amount around here. Since the literal meaning of it is "Too Long; Didn't Read" I feel like it goes against the general attitude that we want on this community.
If the post really was too long, then why not post a comment saying the question is hard to follow because of how long it is and ask for it to be shortened and to the point.
tl:dr; has also been used to say "What I am about to post is long" but even in that case I don't think we should be fostering the use of short hand that the general public wouldn't know.
I am just one person though and would like to know what others think of it.