Kayak licensed/used QPX from ITA back when they were an independent company. Google acquired ITA and as part of the regulatory approval had to operate the APIs for at least the next few years. Kayak continued with QPX, but slowly shifted over to using Amadeus whenever they could. I imagine they've completely switched by now, given how bad the Google <-> kayak relationship is.
That's slightly frustrating to hear that Kayak are getting messed about by Google, as Kayak is about the only flight search tool that I've used that suits the airport situation in the UK.
(e.g. 4-5 international airports reachable within 1 hour drive, another 2-3 reachable within 2 hours drive, flexibility to search across all of them).
The other flight search tools that I've used might, at most, allow one alternative airport.
It seems that (at least the new) ITA Matrix Search can do that as well: Once you select an airport (or city) as origin or destination, it gets added to a list of possible locations.
Being used to Kayak‘s comma syntax, I did not find this intuitive at first. But in fact, it is pretty convenient and powerful.
ITA and kayak were organizationally close in a lot of ways. Kayak brought lots of useful data relationships to the table and ITA brought their technological wizardry. Google was a direct (and hostile) competitor to kayak, who led the anti-trust charge to stop the acquisition. The post-acquisition hostility is just the standard SV playbook stuff like prioritizing Google products in search results and offering amazing incentives to build market share.
I have no particular insight here beyond a general interest in the industry though.
