Can Entity Framework 6 be used in a .NET Standard project?

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.

https://github.com/aspnet/EntityFramework6/blob/release/6.4/src/EntityFramework/EntityFramework.csproj#L6

Some clarification, since EntityFramework 6.3.0 .NET Standard 2.1 has been supported. However there is no .NET Framework version that supports .NET Standard 2.1. Comment from Microsoft:

NET Framework won't support .NET Standard 2.1 or later versions. For more details, see the announcement of .NET Standard 2.1.

This means that a majority of all applications using EntityFramework 6.X won't be able to use the .NET Standard 2.1 project anyway.

https://docs.microsoft.com/en-us/dotnet/standard/net-standard

https://www.nuget.org/packages/EntityFramework/#