It is very much still a thing and the contracts still exist in some form, specifically phone financing and locking. If you finance a cellphone from Tmobile, it will be locked to Tmobile until you’ve paid for the phone in full, which is usually over two years of payments. This is why carriers offer deals on phones purchased through them, and have those upgrade-every-year type plans. The contract has just switched from the phone service, to the phone itself.