Even for niche things like D language, the Discord is the place to go. I learned Scala 3 and D mostly through the help of folks from their Discord servers guiding me.
I would not risk assuming maintainers of an open source project to have the reflex of jumping into Discord to ask questions.
Anyway, D has a forum and an IRC channel. Scala has a Discourse.
The nice thing about forums is that problems and solutions are searchable by other people in the future. I learned many things by myself thanks to this. I would not like to live in a world where you need to engage with people all the time, asking the same questions again and again, to use some tool or some programming language.
[1] https://drewdevault.com/2022/03/29/free-software-free-infras...