I think the sort of Discord group being discussed here counts as "in public". You can join interest groups by just clicking a link with an anonymous account. It's out of view of Google, unindexable, hardish to search and to collect organized information from, but it is more public than say, old newspaper archives in microfilm that anyone can look at in a library.

People are going to discord for that because it is convenient as a platform for a variety of reasons, not because they want to hide it from the public imho.