Discord and Telegram did not find much utility in the open-source community. On the other hand, Slack, which is similar to them, has found wide adoption. This might be related to usage plan, UI and other strategies.
For my experience, Slack and Gitter channels are commonly available to most projects while Discourse finds place in potential and well managed projects.
Apart from these big players, there are several alternatives that I would like to shed some light on like:
Rocket Chat
Bitrix24
There are several other alternatives such as Cisco Spark, Jabber, Mattermost, Salesforce Chatter, Yammer and others. These might have found some use but I have certainly not seen any of these for open-source projects. Additionally, I believe these might not be even a good fit.
GitHub issues are still good and having an additional platform certainly helps contributors and maintainers equally.
I believe platforms like Gitter and Discourse have a sense of openness by default and hence, good to communities that welcome new comers.
On the other hand, platforms like Slack, Telegram and Discord are closed communities by default. One need to be a member to enjoy. Thus, it closely aligns with projects that have company employees working on.
The fact that whether one needs to be a member to enjoy is a critical point that shapes the discussion platform to go for. Personally, I like open communities like Discourse.
It is nice to see that 32.82% of discussion lead to better decision making and changing the original plan. This means that projects are, infact, immersed in the open source community and the community of contributors do have a control over where the project is heading to
