Duome forums - possibly the closest to the classic forums there is. The navigation can look intimidating but it is actually quite similar.
https://forum.duome.eu/
The Discord posted about by lingot in this post has comprehensive organization by language, and over 20,000 members. Great for interactive learning and practicing your language skills.
https://archive.ph/AMnTg
https://discord.gg/langs
Discord organized by mods, recommended to join before the forums close for verification purposes
https://archive.ph/ww3AC
https://discord.gg/8Y32F7ZP
Reddit's r/duolingo. Less organized, but an awesome place for general duolingo chitchat
For educators, the duo teacher's group on facebook (as referenced by HelpfulDuo in https://archive.ph/ezfSJ)
https://www.facebook.com/groups/duolingoeducators