I think you're missing a few words and I'm struggling to fill in the blanks. Set a default what for most wikis? This may also add the ability to reset existing notifications?

Do you mean global notification preferences (https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-echo , but global)?

Apologies for shorthand approach. Hopefully now expanded and clarified.

Isn't this just a special case/suggested implementation of T130655?

They are related, one is the on/off switch for the functionality, whereas this is the settings for the switch. If a user just wants the default default settings, then they are the same thing; if any user wants different settings (from the default default) across all wikis, then they are different things.

@Billinghurst O.K. Pardon my confusion but how, then is this proposal not a re-statement of (nominally already resolved T129077)? Unless I misunderstand this boils down to whether the wiki "controlling" notifications is the individual user's Home wiki, or (always) Meta?

What I am proposing is that at meta that every account can set for every WMF wiki the notification preferences that the account name wants for all wikis. At this point in time if any user wishes to change their notification preferences they have to visit each wikis to make changes. This is not all on/all off settings, this is something like "send email notifications" from individual to daily; it could be flicking the web counter to include or exclude a setting.

I have chosen meta as that has been where WMF has chosen to have global settings be set, ie. default user page, ie. global.js/css.

We probably would want to do this for all preferences, not specially for just Notifications. There is an existing task for this, T16950: [Epic] Support global preferences on Wikimedia wikis