mobile.startup adds a module 'mobile.startup/settings' which depends on mediawiki.storage and adds cookie support.
This is not needed any more given that localStorage is so widely supported.
We don't appear to use the cookie fallback for anything inside MobileFrontend except choosing the default editor (which is strange as VisualEditor is unlikely to be supported on browsers which don't have localStorage)
Note all ResourceLoader enabled clients have localStorage:
https://github.com/wikimedia/mediawiki/blob/master/resources/src/startup.js#L59
So this fallback only assists users who have exhausted localStorage.
I think it is better to use the generic storage library in core.
Possible side effects of this change:
[] Some extensions, such as ZeroBanner use this module. Make sure to not remove the module before swapping out the code in such extensions.
[] It may not be possible to disable a zero banner
[] VisualEditor may load as the default editor rather than the wikitext editor or vice versa
Both are unlikely and low risk on the majority of modern clients.