So some of them might reconvert to 23.976 even when we submit 24 fps masters as per the contractual agreement. Maybe you are right. I will check.
Low budget web series shot at 23.976 even get delivered and played back at 29.97i on some local platforms here in Canada (Crave, Noovo, tout.tv), so anything's possible nowadays.
On the other end, Blurays can be encoded at both 23 and 24, and Vimeo, Youtube, etc all support 24fps. So 24 fps exists, not just on DCPs.
By the way, the loss of quality from going to/from 23.976 and 24 is not much. I've never heard any artifacts from that kind of conversion. But since cinema theatres are most likely to have a better sound system that a home system, I think it makes more sense to have the unconverted mix playing in the cinema and not vice versa.