an option that seems to be surprisingly absent is discussion where the source lives, on codeberg. imo it's very useful to have persistent discussions close to the source.

while i still consider matrix to be the best option for chat on this list, primarily due to federation, it's mostly useful for ephemeral conversations. finding old messages in matrix is often a pain, knowledge isn't searchable (search engines won't see this at all), and even new joiners typically won't have "all" history. structure with threads is also still very limited due to there still being a fair number of clients without thread support.

having discussions in codeberg issues also makes it a lot easier simpler to find context when you e.g. look at a commit, find the associated PR, then see the linked issue with the discussion.

edit to add more info:

what i've also seen in various projects in the past is when more complex issues have been discussed in chat, that a summary and key elements of the discussion or even a copy of the conversation will be persisted to e.g. issues, to allow for easier discussion (real time communication can work a lot faster and more efficient when there is some back and forth) while also ensuring that the information isn't lost over time.

i voted for piefed + matrix, but in reality i'd prefer codeberg + matrix. with federation, codeberg + zulip would likely also be a good alternative.