Hi,
Generally I agree to allow curved traces. But only arcs, not splines or so, because arcs can easily be exported to Gerber (I see no need to approximate them with straight lines).
Currently I also don’t see any fundamental issues with curved traces, I expect the implementation to be straight forward, although quite time-consuming.
The only thing which might be a critical thing is the push&shove router from KiCad, which we plan to integrate into LibrePCB (Add interactive push and shove router from CERN/kicad/horizon). I’m not sure if it works with curved traces. If not, it would be very bad to have curved traces, but no longer be able to use the push&shove router.