Which specific API usage was allowed before and now panics? As far as I can see the only historically allowable API usage that would now panic, is registering exactly the same path twice, which in my books is just a bug.
Certainly it's technically a breaking change, but there's huge difference between making all existing usage of the mux interface incompatible with the new changes, and only making obviously incorrect and buggy usages of the interface incompatible.
