> Has this kind of repudiation ever been tested in the real world? It's hard to imagine a court throwing out email evidence because it lacked a DKIM signature.

They're more likely to ask their expert witness to testify about the evidence, and the deniability of the DKIM signature could be brought up by the expert witness as a reason to distrust the evidence. I wouldn't expect lawyers to discover this argument from first principles.

Has it ever happened? Not that I know.