• Terrible format for archiving knowledge
• Terrible tool for retrieving knowledge
• Locks community access behind a corporate license agreement
• Hands control of community-created content to a corporation
• Prevents indexing by web search engines
• Antithetical to interoperability
• Privacy-hostile

A web forum is far better in most cases. If you can’t manage to run your own, there are plenty of lemmy servers that will do it for you. Even an email list (with searchable archives) would be better than Discord.

If you have collaborative documents that outgrow the forum format, use a wiki.

If real-time chat is needed, irc or matrix.

A project hosting its community on Discord is a project that won’t get my contributions.

Open source inherently means you can compile the code locally,

Open Source means more than that. It is defined here:

https://opensource.org/osd/

If you use the phrase “open source” for things that don’t meet those criteria, then without some clarifying context, you are misleading people.

for free.

Free Software is not the same as “software for free”. It, too, has a specific meaning, defined here:

https://www.gnu.org/philosophy/free-sw.html

When the person to whom you replied wrote “free software”, they were not using it in some casual sense to mean free-of-charge.