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.

Google Flights allows multiple alternative airports as well

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.