OK, I’ll switch to the role of posek here..
To POST something causing someone, even anonymous, pain, anguish, distress, discomfort through words will be Onaas Davar. The fact that you don’t know him/her and he/she doesn’t know you – HaShem knows and keeps perfect score.
The question would be if it’s permitted to belittle or denigrate an anonymous poster OFF LINE, where it would never cause Onaas Davar.
I don’t know for sure, I’d think that too is forbidden.
As pointed out by the holy Chafetz Chaim, lashon hara is in the written word too. And just because the writer is anonymous, there is a real live person who posted, one is belittling that individual Yid / Yiddene. And regardless if it causes harm, belittling, disparaging a Jew is forbidden.
so it seem to my poorest of mind
Froggie the small