How do you think each of these sites should be used?
Back in the day, I didn’t want to make an account for Discourse, so I dumped a bunch of feature requests onto Github. if I were doing it today I’d probably make posts here. I would keep Github for bug reports.
We added the ‘Contact’ section the README to hopefully clarify this.
Basically, github is for bugs and feature requests, discourse for everything else.
The internal dev chat is really just logistical or informal stuff that the general public wouldn’t care about. If anything of substance does come out of there, our goal is to relay it to the relevant github issue or discourse post.
Right now we have, afaik:
For a project that has <10 active discuss-ers in any given week, this is absurd. I don’t want to have to check a bunch of places to keep up with discussions. In particular, I think a ‘public’ IRC should be retired, since it’s near impossible to search for future generations.
I also think we should poll the community on Discussions vs. Discourse and retire the one that is less desired.
Thus:
I also have a suggestion to not use Issues for anything but narrowly defined bug reports or feature development. (Think Agile if you’re familiar.) If you aren’t involved with beets development then you wouldn’t need to check issues either. That would leave just discourse or discussions as your one-stop-shop as an end user.
I didn’t even know there was a Gitter chat 
I agree, no need for IRC, it’s merely a dev flex 
That being said, GitHub Issues is out of scope.
As for GitHub Discussions and Discourse. I’m not sure where we are currently getting more user action.
I think Gitter (or Discord) and Discussions would be my go-to. Discussions would be a more common place for implementation/help I think and Gitter/Discord would be more a community thing.
Seems like most projects use Discord now instead of Matrix, IRC, Gitter, Slack, Zulip, etc.
I will add that Discourse has a nice weekly feed. We’re hoping that GitHub comes out with something similar. (Email digest · Discussion #4075 · github/feedback · GitHub)
I would appreciate more input from maintainers since this is the kind of foundation you need to lay if you want more contributors.
Strongly disagree for the same reasons as IRC. Impossible to search, not on Google. As a bonus, it helps Discord tighten its grip on the open web. It’s not going to be fun in 5-10 years when they decide they don’t want to store chat history indefinitely.
That being said, GitHub Issues is out of scope.
Why?
I’d opt for github discussions replacing discourse. One of the main reasons being it deeply integrates with github. Filing a feature request or bugreport from a discussion is just a few clicks away and everything is nicely linked together then. Besides that, everything is in one place and discourse doesn’t need to be maintained anymore (not sure if this is an issue or it’s comfortable to maintain anyway).
We use discussions as our main message board over at python3-discogs-client, here’s an example of how it looks like if you make an issue out of an existing discussion. It’s linked in both directions automatically: Accessing release catalog number · Issue #74 · joalla/discogs_client · GitHub
The only thing that I think is a drawback to moving to discussions is that a lot of good and valuable information is still only available via this discourse board and thus it actually needs to stay online anway - read only of course. Which means it needs to be maintained still… :-/
My two cents. HTH 
PS: There is an IRC channel for beets? Is it used heavily? Depending on what server it’s on, it might be a good way to bridge it to matrix.org. That would keep the IRC alive and make it possible to have a more modern way of accessing it. Each and every IRC channel I use, I access through Matrix these days. Guys over at matrix.org doing a good job on supporting/bridging over as much as they can together with IRC server admins. Here’s the list of already bridged IRC servers: The official network wishlist for matrix.org / Element · Issue #208 · matrix-org/matrix-appservice-irc · GitHub
Unscientific count of all topics with activity in the last month
I straight up forgot to check Github. Very subjectively I think I like Discourse more but that could be familiarity. I’d definitely prefer Github if it meant having all 50 topics in one place.
I still think the cognitive overhead of checking all these places is not worth it.
A year later after my post above I have to admit that I fully agree with you @RollingStar. Even though I love how github discussions integrates with github, I would rather prefer having everything in one place. If that means it is Discourse I’d be very fine with it, it is a very good communication platfrom as well 
On the other hand I understand why it’s so hard for the beet(box) project to close down one or the other: It would need to stay anyway to not loose valuable information. So one could argue that when searching for information about any beets topic, you should take the time and look through both platforms to make sure the question you are asking has not been asked before on one of them. So in the end it kind of doesn’t matter that there is 2 platforms.
One thing that I’m missing in Discourse is that you can’t mark posts as “correct answers”, I think it’s a handy feature if you want to find out how you can help the project with answering beginner questions for example. Maybe there exists a feature extension or something in Discourse anyway that just has to be activated? Installed?