Check 21
The 2003 US law that let banks exchange images of checks instead of the paper, and made it possible to deposit a check by photographing it.
The Check Clearing for the 21st Century Act was signed in October 2003 and took effect a year later. Before it, the original check had to be physically shipped to the paying bank, and the Federal Reserve ran its own fleet of aircraft to do it. When flights were grounded after 11 September 2001, billions of dollars in checks were stuck on the tarmac, and that became the argument that finally got the law passed.
The law did not force anyone to accept images. Instead it created the "substitute check", a paper printout of the image with the same legal standing as the original. Refusing images became pointless, and within a few years nearly all check clearing was digital. Mobile deposit with a phone camera followed. Americans still write checks; the checks just stopped flying.