Cool topic. From what I’ve seen, the forum was really active around the release of Anniversary Edition (2008) and Global 1940 (2010). Lots of strategy talk, custom maps, and house rules.
Some key posters back then were Imperious Leader (historical setups), Krieghund (rules expert), and Young Grasshopper (strategy vids, tournaments). The Play-by-Forum league was also big, with players like Cow, Baron Munchhausen, and Elk.