Hi Michael,
As I've already commented in
https://gitlab.gnome.org/GNOME/geary/issues/403, having a "mainline" branch instead of a "master" branch is confusing for applications like Damned Lies, breaking for example cherry-picks from other branches to "master" branch, as it doesn't exist.
Stable branches can be named with your own nomenclature (no need to name them gnome-x.yy) but I would encourage you to keep the "master" branch. It is the standard name for GNOME's development branches and helps developers and contributors to identify where to look up the last version of the source code.
If every GNOME's module begins naming it's "master" branch to whatever the maintainer wants, this will be a chaos... I don't know where is the advantage of this renaming (we don't belong to Linux kernel develpment, as exposed in #324), but coherence across modules should be kept.
Regards.