On Evergreen, we currently test on .NET Framework 4.7.2, .NET Core 2.1, and .NET Core 3.1. In some instances (e.g. netstandard2.1 on M1) we test on .NET 6. (We also test against .NET 6 on OCSP but that's more happenstance than intentional design.)
Given that .NET 6 is a LTS release, we should run our test suite against it. This does not involve adding net6.0 TFMs to our production code. It only involves running the netstandard2.1 assemblies in the .NET 6 test target that was added in CSHARP-4189.