Doesn't it feel like this is happening more & more?
I admit I don't have hard figures, and I'm not sure how I'd get them, but my strong feeling is that things have been getting progressively better.
That is to say, more of the hardware the OS actually cares about is class compliant rather than requiring custom drivers.
There are big holdouts. Networking for example, Ethernet cards are hot on compliance with a well documented standard right up until you reach the controller chip, I guess all the standard-friendly people at network peripheral companies work over on the link-layer side of the operation.
But on the whole I can't think of any category where things are less class-based than they were in say 1995, and many where things are more so.
I thought we'd missed a trick twice with USB, firstly sound (but today everything is either USB Audio or HDA on PCI) and then webcams (but the USB video spec didn't die, it was just a long pregnancy and today you can walk into any store and buy webcams you've never heard of that work in Linux)