Sirs.
I have no idea what setting I have changed, but all of a sudden, all new tracks are assigned to the metronome bus.
I can fix this by reassigning each track to the master. But that is not ideal. Anyone have an idea?
(Also: even if I mute the metronome bus, I still hear it while recording)

Hi. Right click on the bus you want as your default, and choose make default bus.
All the best.

Thanks, Ken!
I also found that this is a project setting.

Yes and like all per-project settings, making a change to the project templates causes new projects made from those templates to inherit the new default settings. AFAIK, all the project templates bundled with software have a predefined master bus which is different from the metronome bus.