> I know every name on mysql devel team.

That's one project. You did not know every name, or most names, in FOSS generally. Today as well - a project which uses GitHub still needs a mailing list, web forum, chat channel (Matrix/IRC/discord/Telegram/whatever) to discuss and coordinate, and that hasn't changed.

As for whether signal to noise ratio on mailing lists was high - that really depends on the list. I don't see that much noise on GitHub repositories, to be honest - it's not easy to post noise that many people will see, so there's not much motivation for it.