I will let @hugh the PM of staff-experience answer too, but I don’t really see a problem with 3 different ways of finding things, as they are not all doing the same thing.
Admin search is the most comprehensive and global. The other two (sidebar filter and settings filter) are more localised filters that can be helpful on a page-to-page basis. Maybe the sidebar filter could be removed at some point since all the links are in admin search, not sure.
I know the team does have plans around what to do for “All site settings”, but I don’t think there is a need to remove the filter from any of the other filtered settings pages, e.g. Legal, at this time.