PSD2
The EU's second Payment Services Directive, which forced banks to let third parties in through APIs and introduced strong customer authentication.
Directive 2015/2366 was adopted in 2015 and applied from January 2018, replacing the first PSD of 2007. It did two things that reshaped the industry. First, banks had to give licensed third-party providers access to a customer's account once the customer consented. That covered both reading balances (AISPs) and initiating payments (PISPs). This is the legal backbone of what got marketed as open banking.
Before PSD2, aggregators solved the same problem by asking for your online banking credentials and scraping the site, which is about as reassuring as it sounds. The directive's other big change was the requirement for strong customer authentication. Its successor is already on the way: the Commission proposed PSD3 and a separate Payment Services Regulation in 2023.