Discord is not a replacement to the forums.
It's a way to get in touch with community members in mere instants and get immediate feedback on a number of quick queries, but it doesn't in any way replace a public release thread, an eyecandy thread, a project board, and so on. Discord content is also at best quite volatile, relegated to the people who were paying attention to it during a given time span. I dare anyone to fall 150-200 messages behind any non critical Discord conversation, and read all of the messages he/she missed. And on a regular basis, I may add.
The forums are open to the rest of the world and are easily accessible to those who don't have an account. It's the best tool to get FreeSpace modding promoted and viewable to anyone willing to take a look. This can be determined by the number of views and guests HLP gets on a daily basis, and the fact that - at least in my country - there's an interesting number of players who lurk and play the mods without creating forum or Discord accounts. The forums are also the community's "healthometer" as perceived from such guests, who judge the degrees of activity of HLP based on public content, not volatile chat between a small and selected number of members.
As I clearly stated elsewhere, it's ok to have both supports because each one of them has its uses, but dropping either because "no one reads it" is a bad move, IMO.