People use Related changes to monitor changes on all pages in a given category. The plan is to watch all pages, but also all changes on their talk pages. That's not possible.
How to reproduce:
- open Related changes and filter a category
- filter to have only talk: namespace displayed
- make a change on a talk page of an article listed on that category
Expectation: the talk page change is listed on the results
Reality: no results are displayed
If you add a category to the talk page, that talk page will be listed and filterable.