I don't think that's what the question is about. No, you can no longer actually enter the airport codes, you do need the drop down menu, regardless of whether you type the airport code or the first three letters of the city. After you enter three letters, you get a list of airports that match somehow and you select.

If you don't get the menu, you can't select and you can't enter anything actually.

It's part of the continued effort from AC to make their page more fancy, more complex and less resilient. Less resilient to both problems on their end and at the customer's (oops, I don't think they understand that word?) end.

As in the current case. (BTW, I know because the same problem happened to me once. Although closing Firefox and reloading made the problem go away.)