Task declaration supports a Runtime attribute that allows MSBuild-running-on-.NET-4 to call tasks that support only .NET 3.5 (by spawning an MSBuildTaskHost.exe that runs on .NET 3.5 and communicates with the rest of the build via IPC).

Similar support could be used to support running .NET Core (or .NET Standard 2.1+) tasks from Visual Studio with Runtime="NET" (or something like that).