IPv6 is too complex, error prone and unsupported to deploy without shooting yourself in the foot, even now, a few decades after introduction.

Which is purely down to people not testing things before releasing them, because the support is there but there's layers of unnecessary stuff put in the way. Like I had an old ISP provided router that ran Linux, but the management UI was only ever tested against v4 networks so none of the v6 stuff was actually hooked up correctly.

Support in desktops and mobile devices is effectively 100%, but even in embedded hardware there's often full support, just not enabled correctly or tested.