Users are very interested in notifications that pertain to their User talk pages. To cater to this interest, a User talk pages filter option on the Notifications Page left nav surfaces only those notifications that pertain to the users talk pages.
This option works X-wiki. Which means it should roll up results from all the user's user talk pages across all wikis. It should apply equally to Flow and non-Flow User Talk pages.
My understanding regarding this was that it was hard to achieve an aggregated list of notifications about all your different user talk pages in a single list (especially when displaying read items).
Even if we cannot provide all information combined in a single place, it was considered feasible to break it per wiki and even surfacing the number of unread notifications to provide at least a global overview. I captured some ideas about this in the discussion on T129366#2209474
The concept for this feature has been simplified to avoid the issue referenced above, and this ticket is no longer necessary. I'm marking this declined.