You can route DDR in KiCad and there are some online reports of people doing that. KiCad is not as nice as the paid software and there are shortcomings but I think 'pretty useable' covers its status accurately.
I think that's hyperbolic. There are several designs advertised as being designed in KiCAD that have DDR which means there are way more not being advertised.
I wouldn't describe this as a "pretty usable" means, however, for the same reasons KiCad is unusable for DDR routing.