It never was and is never going to be a goal to use CoreFX tests to find bugs in .NET Framework. Really their only value is as documentation, where we intentionally changed behavior, we created two similar tests next to each other. This is an expensive kind of documentation, that runs every day, inevitably fails from time to time (given how many there are, and the fact that .NET Core is more reliable now). It would be enough documentation to have a comment, or the mere fact that a test is excluded from .NET Framework runs.

There is still some value in having .NET Framework runs, and making new tests default to running on both. But there is a cost too, and in future more and more often, new tests will not apply to .NET Framework because new tests will mostly be for new API and new behaviors. Several times a week, someone writes a test that fails in CI only on .NET Framework, because they didn't think to exclude it. It would be nice to change the default, so new tests don't run on .NET Framework, but I expect that would be too fiddly (involve marking all our existing tests somehow). At some point we will probably want to stop the .NET Framework runs entirely, relying on the tests we have built up (and taking care when we change their assertions) but perhaps that's going a little too far for now (?)

Suggestion: make a pass and delete all tests and test branches that are .NET Framework only. Thoughts?

@jkotas @safern @stephentoub @ViktorHofer @ericstj @marek-safar @hughbe