Craft suddenly no longer works on Heroku
This bit us a few days ago, as well.

As far as I could tell from the build log, the cause was that Heroku bumping their PHP version from 7.3.x to 7.4.x, and exposing an incompatibility with one of Craft's dependencies.

To lock your PHP version to a 7.3.x release on the Heroku stack, just add this to your composer.json:

{
  // ...
  "require": {
    "php": "~7.3.0"
    // ...
  }
  // ...
}