"What is this movie about?"
IMHO I cannot agree that's grammatically correct. Of course if someone says it without 的 I won't be mad. In order to speak natural sounding Chinese such tiny bits are somehow important.
Whenever we say A 是 B, both sides of 是 should be a noun, a clause or an equivalent. We cannot use a noun determinant.
这支笔是红 is wrong
we can only say
"this pen is a red one"
for saying "This pen is red".
Compare these correct sentences:
You are correct that my statement was inaccurate. As is suggested in your examples, the 是…的 is probably in fact an A 是 B sentence structure with the B noun phrase elided. You will sometimes see 是 omitted in such sentences, but not 的. I probably should not have posted so late at night! Thanks for correcting my error!