Maybe this is an oversight? While "Edited [date] time" appears in the New Discussions forum, when opening the same discussion in Speedgrader, that information is missing.

My TA comments on all discussions prior to the deadline and students are encouraged to edit their posts to get a better grade but only some students do this. I use the fact (or lack of) editing to give students coaching when I'm grading in Speedgrader. The old discussions had this information present, it is missing from new discussions.