Page has been renamed.
This help page should be moved to "Help:Notifications/Notification types" (singular "/Notification") or, better yet, just "Help:Notifications/Types".
(I'm not going to try to do this myself, since I don't know how page moves work [or maybe don't?] when the corresponding talk page is Flow-enabled.)
Simple curiosity from a non-native English speaker, what is the grammatical reason for renaming it?
Just jumping in here, if I may. The current wording is not idiomatic.
Adding 'type' to a noun creates the noun phrase "[noun] type", with [noun] always being in the singular. If there are multiple types, the noun phrase becomes "[noun] types", again with [noun] in the singular.
So, for example:
Types of cell -> cell types
Types of hair -> hair types
Types of property -> property types.
But anyway, Help:Notifications/Types would be best.