I believe AT&T still has some options, but Verizon has been changing theirs where it went from 50% to 75% and then last year they did away with that completely and just basically have the installment plan where you simply pay it off and that's it (no trade-in or anything like that).
You're behind the times. Verizon will allow people to upgrade from a 6s to a 7 after a year and 50% of the phone is paid off (though you must turn in your old phone):
http://www.cnet.com/news/verizon-begins-offering-annual-iphone-upgrade-plan/