@Rbirmann There's a new draft documentation page explaining how to create your own notification types, currently located at https://www.mediawiki.org/wiki/User:MSchottlender-WMF/Creating_a_new_notification_type (to be moved into the main docs once more complete).
Hopefully that will help you program your own new notification. :)

@Quiddity unfortunately my php programming skills are on the "hello world" level. Also my familiarity with MW code is close to zero. The article you sent is way beyond my capabilities, I am afraid. :(

Just a brief note, this issue is a problem for seasoned Wikimedia project editors as well as non-Wikimedia users of MediaWiki software. For instance, several experienced Wikimedians designed the Wiki Global Check-In, an event organized for community members during the COVID-19 pandemic. The original instructions had users "pinging" one another on a page in the main namespace on Meta Wiki; but even these seasoned wiki editors did not notice until far into the planning that the "pings" were not resulting in notifications. (See the talk page as well.) https://meta.wikimedia.org/wiki/CheckIn

I do not believe there is a downside to having the addition of user pages to ANY wiki page result in a notification. If there are cases where this would result in a flood of notifications, perhaps a preference could be established that allows individual users to turn of notifications for specific namespaces (or by other means).

On second thought, this may not have to do with namespace. I'm not very familiar with what does or doesn't cause notifications to fire (which can make communication a little difficult). The example I cited above seems to have to do with requirements of using proper signature code...and using an updated signature. See task T66778.