Chat has a way of getting completely lost. All your knowledge that goes into chat either goes into somebody’s head or it just disappears into the ether. This includes Slack, Discord, Teams, etc. Useful as a secondary channel but serious projects need something more permanent.
Bug tracking systems just don’t support the kind of conversations you want to have about things. They’re focused on bugs and features. Searchability is excellent, but there are a lot of conversations which just end up not happening at all. Things like questions.
That brings us back to mailing lists. IMO… the way you fix it is by having redundancies on both sides of the list. People sending messages to the mailing list should send followup messages. You should also have multiple people reading the list, so if one person misses a message, maybe another gets it.
Mailing lists are not perfect, just better than the alternatives, for serious projects.
(I also think forums are good.)