I honestly don't know if there's a dependency with other packages requesting this update in core repository, but there surely is in the Pantheon stack.
I obviously know how Solus core is not interested in adding / helping keeping Pantheon support in any way, but it'd be nice to have this package update, which does not bring any retrocompatibility breakage anyway, according to the Git history - exception made for the Services.Settings which is now deprecated in favor of GLib.Settings.

Link: https://github.com/elementary/granite/archive/5.4.0.tar.gz