That's an interesting thought.
I can definitely see the gating being a problem down the line, although there are solutions to this (one that comes to mind is publishing channel archives in some indexable format periodically).
The "why" feels flow-related to me. The message board format isn't super well adapted to active discussions, but real-time chat on spaces like Discord and Slack feel snappy if you want to have a "chitchat experience" with other devs. I wouldn't necessarily call it a new thing though; before Discord and Slack, IRC was a pretty popular way for OS communities to organize, and that too suffers of the same lack of indexation (unless steps are taken, i.e. publishing archives to make them accessible to those outside the chat server).
From the perspective of someone who runs some public-facing OS-related Slack / Discord spaces, my approach has always been to try to get folks to create issues on projects' GH tracker when it looks like other folks would benefit from the insights or to organize reports / requests. That way, while the initial chat happens in the chat spaces, there are artifacts that are also available outside of them so that others can engage without joining the chatrooms.
To the last point about repeated support request -- questions asked should always drive documentation. Whether it's on Discord, GH Issues or some forum, if a question is asked repeatedly, it should end up in the FAQ / troubleshooting docs. That feels more like an organizational problem than a problem with the platform (case and point, StackOverflow is a treasure trove of questions about everything and anything, but there's active moderation to weed out dupes despite everything being indexed, repeats will happen regardless of previously asked questions being available or not).