The old mantra goes 'don't put anything on the internet that you don't want to read on the front page of your national newspaper - we encourage our colleagues to reflect carefully on what students should or shouldn't be encouraged to put online, anonymous or otherwise.
It used to be the case that 'anonymous' meant that students were anonymous to each other, not anonymous to the instructor,
Even if t were anonymous, you would be able to match the date and time of the post to 'New Analytics - participation' or to the Canvas user access log - so there is no true anonymity in most LMSs - the students can run, but they can't hide 😉