Ain't no expert, but my understanding is that on a long-course (Ironman) it's never permitted. On Olympic course it depends as you said, but generally it's not permitted for amateurs (even at ITU events). I think it's not feasible to implement "no drafting" on a lot of pro races, which have a short, spectator-friendly course with lots of turns. Drafting is nearly unavoidable. Considering the big prize money at stake, it would lead to nasty arbitration issues!!

Otherwise, my understanding is that most see triathlon as an individual, near time-trial event.

I sort of hope that drafting is never allowed in amateur races. Currently, it's the only way (beyond the odd time-trial) where amateurs can race bikes - pretty much any bike - without too much risk (pelotons are very dangerous, especially for the untrained).