This used to be true, but since Qualcomm bought them out, they have done a complete 180 and basically reverted to totally closed designs (closed firmware, closed drivers, no spec sheet). The 9xxx chip-sets (which are only 802.11n) are the last ones designed before Qualcomm took over, the 10xxx chip-sets (802.11AC+) are closed and completely locked down, you cant even change your MAC address its all stuffed into a giant psuedo-driver firmware blob.

Are there any "open" 802.11ac chipsets? A quick search didn't find any.

No, closest thing is the mediatek chip-sets that at least dont enforce wifi parameters via firmware (fullmac vs softmac).

Things are getting worse, not better, on many fronts when it comes to user freedom and control over their devices. Apparently many responsible engineers in this area lack the ethical consideration of their work. The more I see this, the more I worry about bridges, airplanes, and every other piece of technology that is developed by engineers that may or may not have ethical principles.

Good thing the people building bridges and airplanes are Professional Engineers and everyone can call themselves anything they want except for P. Eng.

Do you have any examples of useful devices?

Well that sucks.