MSTest.TestAdapter is intended to be used in actual test projects, not in helper projects/libraries. Actual test projects aren't supposed to be netstandard2.0.

The only thing we lose by this is incompatibility with Xamarin projects (e.g, MonoAndroid12.0). But as that's out of support already, I think a break here makes sense.

https://dotnet.microsoft.com/en-us/platform/support/policy/xamarin#microsoft-support

Xamarin support ended on May 1, 2024 for all Xamarin SDKs including Xamarin.Forms.

The only way to have a test project that can run and targets netstandard2.0 is #3984 (comment) which I think is uncommon and isn't something we should support.