Dropped
We standardized on xUnit — it's closer to the .NET Core idioms and lives better in that ecosystem. I'm shelving MSTest with no regrets: its "works out of the box" duty was served back in the ASP.NET MVC era.
Assessing
Visual Studio's built-in test framework. Started trying it out for covering business logic on ASP.NET MVC projects. Works without any extra packages, which is handy in a corporate environment where every dependency has to be approved.