Erik Darling shows how to make filtered indexes a bit more robust:
But why oh why didn’t SQL use my filtered indexes for even smaller subsets of the filter condition? It seemed insane to me that SQL would know the filter for the index is on (x > y), but wouldn’t use them even if (z > x).
The solution was to put the filtered column in the include list. This lets SQL generate statistics on the column, and much like getting rid of the predicate key lookup, allows you to search within the filtered index subset for even more specific information.
Filtered indexes are as useful as they are mercurial.