To the best of my knowledge, this is still a valid project. I do not believe EF Core has changed their OptionConverter interface since it was released. If this library needs attention, please open an issue!
A value converter that maps nullable database columns to F# option fields/properties
This is a NuGet package of the code described in a recent blog post, which provides for mapping nullable database columns to F# options in the entity's fields or properties.
The blog post describes how to set it all up; the test project here demonstrates it (along with SQL to ensure that the results are what they should be).