You are correct that 1.0.38 is that last version of the SDK that supports .NET Framework. If you look at the dependencies for 1.0.38, the you can see that the Standard 2.0 version allows for Newtonsoft >= 11.0.2. This means you should be fine, but nuget is choosing the .NET Framework version, and because of that it thinks it is still locked into v9.0.1.
The recommended path is to update to .NET Core or newer, but I understand that's not necessarily an option, which is why in the last few versions of the 1.x SDK the version was opened up.