Historically there has been resistance to taking on ChainRulesCore as a dep. I’d assume this is no longer a problem with package extensions, but I can’t speak for the maintainers.
Ok, after reading up on this a bit I think I now understand the issues a bit better. Actually it was kind of expected to run into this kind of problem. It seems that package extensions were in part introduced to make it possible to avoid this, but the ecosystem hasn’t yet fully caught up with this.
So if the adjoints were moved to Distances.jl as a package extension, would that be backwards compatible with Julia <v1.9?
there are 3 different ways you can make it backwards compatible depending on which 1.9 behavior you want.