I've tried that in my own community but it's a real struggle because in my experience newbies drive the conversation much more than pros so in my case gatekeeping was creating a silent community of highly skilled lurkers.

With no gatekeeping, there's lots of activity and pros feel like participating once in a while with deep discussions sparked from seemingly simple questions, but on the other hand a lot of pros feel alienated and leave if the average level of competency is too low.

I've been toying with the idea of pro:newbie ratios but haven't implemented anything yet.

An approach I think is good is for pros to help visitors solve problems, using messages of interest to other pros who can also participate and it can really brainstorm in unforseen ways.

Pros learn by watching other people's problems get addressed, and contribute when it falls under their domain expertise. They interact in a positive way to form the personality of the messageboard. Which should be a reference point for an actual newbie who wants to participate long-term, but it should be expected that most visitors will not return after their problem is solved. The message board becomes a curated/community archive for solutions.

Where the subject matter and the discussion are contained within the board.

That's for a problem-solving message board.

Now HN is a tech news aggregator with comments which make it a board of a different kind, with way more diverse comments to deal with than any one news site.

And very many news sites individually are challenged to adjust to the recognized lower quality of comments the last few years by trying experimental approaches or no longer accepting comments at all.

The news is always elsewhere, only the comments are here and that makes it debatable.

Maybe there's such a thing as a problem-creating message board?

You would need to provide professional indemnity insurance cover. Most policies likely don’t cover advising the whole internet.