Did the USB 2.0 BC "charging downstream port" never catch on? Has a major computer manufacturer ever implemented it?
Texas instruments makes a chip BQ24392 that can detect if the host is a CDP, SDP, or DCP.  

I think the way to answer you question is to get the BQ24392EVM (or similar) and plug it into a bunch of computers and see how many of them support CDP (you will know if the indicator light comes on).

https://www.digikey.com/products/en?keywords=BQ24392EVM

I have personally plugged that eval board into a various computers in my house.  I found that older computers tended to have less support than the newer ones.

On my Toshiba Qosmio X70 laptop one or two of the ports support CDP and the others don't.  I also have an Acer laptop and it has support, again on only some ports.

Usually I have found support to be limited to the blue USB 3.x ports.

If you design a product that relies on the ammount of current provided by the CDP spec then you need to have a contingency plan because your user has a very good chance of plugging into a port that doesn't support CDP.

In my case I used the BQ24392 to inform my device of the host capabilities and limit my power useage acordingly.  But if you really need the higher current, you could also use that knowledge to indicate to the user that they need to find a better port.