Got to disagree here, r/AskHistorians is a top-tier academically-minded Q&A forums, and their wiki is possibly one of the best resources on the current internet for layman interested in history. They are not necessarily the
best Q&A resource (MathOverflow maybe?) but worth a mention. This is not to say AskHistorians is very forum-like, they now moderate the answers with the spirit of journal editor.
In general, there are good subreddits that sometimes develop community spirit. Unfortunately they are a fleeting phenomenon: either they grow too big, or die off (or in the rare case of AskHistorians, become something that can manage the flood), and if you don't find them by accident, it probable one heard about them because they are becoming too popular.