Before I have brought up my dislike of finding FOSS projects using Slack as a primary communication channel. I also want to express disdain for projects using Discord as well. But I would like to go one step further.
No FOSS project should make a proprietary service a primary communication channel, ever. It excludes those who do not want to risk their privacy being exposed, excludes those to do not user proprietary software, and does not fit with the principles of something like Mastodon.