I use EF 6.1.x Code First.
I have read that an Index with Filter Expression is not supported by EF latest.
There is also no solution on SO:
EF 6.1 Un
Please note that right now EF core 2.1.X added built in support for filtered indexes via the HasFilter extension on the IndexBuilder, so a custom implementation is not required anymore.
HasFilter
IndexBuilder
See this for more details