Eitan Blumin looks at filtered indexes:
In fact, absolutely no functions of any kind can be used within the WHERE clause of a filtered index. Not even schema-bound user-defined scalar functions.
Unfortunately, as stated in the Microsoft Docs page about Filtered Indexes, the WHERE clause of a filtered index can only support simple comparison operators.
Well, it’s not entirely true, as you CAN actually use some functions, but on two conditions:
Read the whole thing. Eitan lays out one limitation of filtered indexes and provides a couple of potential workarounds.