The primary channel for important development-related discussion is the scummvm-devel mailing list. All developers should subscribe to it.
There is one note about scummvm-git-logs and scummvm-tracker. Consider them as a read-only lists, i.e. if you have comments/questions on someone's commit, post your follow-up to scummvm-devel.
The full set of public lists are:
This is the official developer mailing lists, for planning releases, and general discussions concerning development and project management.
Commit logs, receiving an email for every Git or SVN commit. (The "cvs" in the name is for historical reasons, and only kept to avoid the hassle of moving everything to a new mailing list.)
Notifications of all new bugs, patches and feature requests. If you subscribe to this one you'll just be notified about new tracker items. If you want to keep track of an arbitrary tracker item you have to use the "Monitor" option/button on the page of that tracker item.