@gotwf's claim that Ruby on Rails is somehow history is absolute nonsense. So is the claim that it's somehow hard to customize and scale.
Most people who make such claims have no idea what Rails actually is. They've never used it, or at least never developed anything substantial with it. In addition they've often spent most of their lives learning something else and is therefore reluctant to vouch for anything new regardless of what it is.
Having said that, I think Discourse is crap. Way to many features, and from a Rails perspective it's an absolute mess. Plus, the fact that the community refuses to help you unless you run Docker is just sad. NodeBB is by far the better alternative. However, if you want to see a Rails forum done right, check out Thredded: