EMV - Euro, MasterCard, Visa is a technical standard for "Smart" payment cards and for all payment terminals and ATMs that can accept them. The first EMV or chip cards were introduced in 1993 in Europe and have slowly become the industry standard. In 2014, the United States began its migration to EMV and, once again, merchants across the country had to update their machines and systems to accept the new chip cards.
Merchants weren't required, and are still not required, to adopt EMV technology. New machines cost money and the processing cost is the same whether chipped or swiped. There's also no fines or penalties if you don't upgrade. So why bother?
The US is a big ship to steer, and the likelihood that most merchants would drag their feet rather than buy new equipment was pretty high. The idea of charging fines or penalties would just be a mess so the industry had to become creative.
Rather than fines or penalties, the industry announced that starting Oct 1, 2015, there would be a "Liability Shift" for anyone not using EMV technology. Essentially, this meant that if you swiped a chip card and that transaction was disputed, for any reason, then you automatically LOSE. No fighting it or defending yourself or sending signed receipts; you just LOSE. The industry also imposed requirements on the processors to get it done by certain dates, etc.
This worked, and most merchants adopted the technology fairly quickly.
Note: Pay at pump gas stations deadlines have been extended several times, the last deadline was April of 2021 but by then only half the stations nationwide had been updated.