As you likely already know these forums are long depreciated and mostly an archive. However as the new freegamedev.net forums don't work that well for hosted forums, we (freegamedev.net admins) therefore didn't want to lock the old forums completely yet.
In the last couple of months the Stunt Rally team did test out our alternative hosted group forums running on Loomio. There forums are more developer oriented, which is also why they run on our sister project f-hub.org and not freegamedev.net.
As spam issues and other problems with these old phpBB forums continue to mount, it is necessary to find an alternative. The best we can offer is the above mentioned Loomio group forums and I took the liberty to create a test one for SuperTux here: https://groups.f-hub.org/supertux/
It would be nice if one of the official SuperTux forum mods could test it (let me know once you registered so that I can promote the account as a owner of that Loomio group) and give some feedback.
I think most of the general player chit-chat has moved to chat (Discord and IRC etc.) these days, so focussing more on developer talk with the forums is probably a good idea. Loomio also has a nice integration with Discord and a few other chat services for notifications.