Imagine being in a corporate environment trying to implement an OSS into your platform and having to tell your 50 yo teammate: "Oh yeah, just pop in this Discord server real quick to see any relevant info". Instant credibility loss
The loss of credibility is not because it's discord,. specifically.
It's because the project thinks a chat platform is an appropriate way to document a project. I would feel the same way if someone told me to get on IRC for docs, or Slack.
To be fair, I could say the same, but is probably a biased sample.
I have other red flags, like only distributing on docker, that I've tried, and tried again, and found that it's a sign of a badly run project. But I can't state any confidence on the discord based rule, because I've never tried to make any run.
The docker thing really grinds my gears. I see it as the ultimate "works on my machine" mentality. Basically they can't be arsed to write software that is robust to changes in hosting platform.
The Gagguino project is a counterpoint to this. They have some extremely limited documentation, but to really build one you probably are going to need to dig into Discord. I hate it. The project is really cool, though, and I'm building one right now.
Its the same as the GitHub problem though, if you want to get community involvement then the necessary evil is to go where the people are. We use GitHub and Discord as that is where the vast majority of our users are, our Lemmy community sees barely any activity over our subreddit, we have barely anyone clamouring for Matrix or IRC. Our Mastodon is probably our only large 'fedi or fedi—adjacent' platform and thats because we drew the line at twitter. Would I love to get away from Discord? Absolutely, but that limits our ability to have an active community whilst we are still growing the project.
This is often done by people while the project is unstable. No need to write documentation that gets outdated every few weeks, when you can help people live in discord.
D*scord is technically searchable and fairly archiveable (messages never get deleted due to old age (in my experience at least) or if the original poster deletes their account). And some d*scord servers even have a Q&A mode similar to st*ck *verflow. But yeah, not the right tool for the job, not to mention ABSOLUTELY PROPRIETARY
Zulip is a little better in this regard. I'm involved in Lean, which uses Zulip as the primary mode of support and documentation. While it's usable, I still think that a Discourse style forum is the way to go.
Or if you find the project a while later, and the link/server is dead, either because the maintainer forgot to update the link, or the server shut down/removed invites for some reason, like spam prevention.
There needs to be some plan to migrate to stable documentation at some point though.
Hell, even a small traditional forum is better searchable.
What I see happen is that the people with the knowledge get so busy answering questions in discord that it impacts the efforts on documentation and on the software itself.