Matrix is a better platform for realtime communication, but it has the same issue with needing an account and being difficult to search. Any discussions that take place on Discord or Matrix will be fleeting, as it prioritizes only the most recent discussion in the chat. Thus making long form discussions about particular topics impossible.

All technical discussions should be archived on a searchable forum. If you are using a source forge like GitHub and GitLab, then public discussions should take place there. There's no better place for discussions and questions about code than in the same place where the code is hosted itself. Platform integrations make it very easy to associate discussions to commits and merge requests.

While not ideal, even hosted forum platforms like Lemmy and Reddit are still better than using a chat client. If only to serve as a platform for broader public discussions and questions. People are more likely to already have a Lemmy or Reddit account than they are to have a GitHub or GitLab account.