xmlunit
XMLUnit 2.10.0 doesn't work with NUnit 4.x
I was thinking if the Description could be set there would not be an issue. In the constructor for the static calls you could force a description but that is a breaking change, but it would work. The only other way is to add a different instance constructor that allows setting the description and then remove the static Instance calls. Again breaking change
Worst case I'd create a new constraints-nunit4 project if we cannot find a reasonable way to make things work for 3.x and 4.x at the same time.
Right now I'm struggiling wth my local setup since I need an oldish Windows VM for the ancient framework versions XMLUnit still supports. It may take some time until I can really take a deeper look - in particular since time is something I currently lack anyway.
I've been trying several approaches, the problem is you need to have the setter in order to override Description in NUnit 3.x - and at the same time the setter makes the override invalid in the NUnit 4.x case.
What I've got so far (in main, I really should have started on a branch and will continue experimenting there) is a working new constraints library for NUnit 4. NUnit 4.x targets .NET 6 and .NET Framework 4.6.3 as minimum and I'm currently struggling with finding an approach that doesn't force me to have even more solution files or manually execute single project builds.