And if not, will it be? If so, when?
I haven't been able to find any clear official information on this, and the limited information I was able to find on Stack Overflow is a year or two old.
I was able to find official information saying EF 6.3 will be able to be used in a .NET Core project, but I couldn't find a release date for it, nor am I sure what the implications for use in a .NET Standard project are.
EF 6 supports .NET Standard 2.1, which is currently supported by .NET Core 3.0 or later - no .NET Framework version. EF 6 also supports .NET 4.0 and 4.5 or later.