Also, interaction with a Free and Open Source project ideally shouldn't require use of proprietary software.

Some examples of major open source projects that don't insist on Discord:

* KDE offers its own forums, mailing lists, IRC, Matrix, and, yes, Discord. [0]

* Gnome offers its own forums, and Matrix. [1] Its old mailing lists are archived but now closed.

* Postgres has mailing lists and IRC [2]

* The Linux kernel has mailing lists [3]

[0] https://community.kde.org/Get_Involved#Many_Ways_To_Reach_Us

[1] https://welcome.gnome.org/

[2] https://www.postgresql.org/community/

[3] https://lkml.org/