Beginning at 21:39 UTC (17:39 EDT) on July 17, 2020, a CloudFlare incident caused widespread impact to many Internet services and customer applications.
This appears to be materially resolved as of 22:09 UTC (18:09 EDT).
Heroku does not use CloudFlare for its services, but customer applications that use CloudFlare for DNS or CDN services may be impacted. In addition, apps that make API calls (client- or server-side) that are hosted on CloudFlare may see a cascading impact.
For details, please see https://www.cloudflarestatus.com/