Agreed. No need to support it in older versions. Moreover,
I'm trying to add another option to join project and vc indicators
on the mode-line that won't be possible to support in older versions.
Then that needs to be explained in the docstring. project.el is installable
in older versions.
Hopefully I explained this sufficiently well in the docstring
both for users of older versions and users who customized the
default mode line.
Not sure: I imagine that most users don't customize mode-line-format
themselves, and thus don't know which contents it has.
I would append some clarification that in Emacs 30 the mode-line has that
element by default.