By this standard, no free software project should use the Internet at all. Unless you jump through the kind of hoops that even smart, tech-savvy, security-minded people don't, someone is tracking most of what you do. Your ISP can track your DNS requests unless you use DNS-over-TLS (no on does); they can track every IP you connect to unless you use a VPN (in which case your VPN or upstream provider can); in most cases your email isn't secure (no one encrypts email, and eventually it's unencrypted somewhere); communication via alternate mechanisms suffers from the same problems as Discord (IRC, Slack, and other services can also be monitored); every search engine can track you. I could go on.

The bottom line is that Discord doesn't stand out as a particular offender here, and the steps required to alleviate the author's concerns are inconvenient. Most projects have more important things to worry about with their limited resources.

Communication secrecy simply isn't a priority for most open source projects. In fact it's antithetical to the majority of open source goals. Someone pointed out that users might want to discuss security issues privately, and while there's a little bit of merit to that, it's a contrived example. It's not one of the author's concerns, and it's not an issue that arises with most projects. Open source development benefits from being done openly, and secure communication isn't a priority.

I'm in favor of using open source, private, self-hosted tools whenever and wherever practical. I'm not defending Discord specifically, nor encouraging its use. This author is just hung up on something that most people don't care about, especially in the context of software that's meant to be open and public.

There's something about this sort of obsessive fixation on a non-issue that makes people roll their eyes and not want to work with you. People who are using their limited free time to work on some random small project don't want to be lectured at over principles they don't share.