Adam Engst, TidBITS:

In many parts of the US, it has long been possible to make local calls with just the seven-digit phone number; the three-digit area code was implicit for calls between numbers using the same area code. Last year, however, the Federal Communications Commission decided to establish 988 as an easy-to-remember three-digit phone number for Americans to use to reach the National Suicide Prevention Lifeline in times of crisis.

And:

Since some areas that use seven-digit dialing also use 988 as the first three digits of the number, the FCC is requiring all carriers to implement ten-digit dialing in those areas, with 24 October 2021 as a cutoff date.

10-digit dialing started becoming common in the US in the 1990s, with the ability to only dial 7 digits within an area code slowly getting chipped away. This is the final nail.

Here’s a link to the 10-digit-dialing Wikipedia page.

And here’s a link to the current list of area codes that still support, at least in part, 7-digit dialing.