This is a re-post of https://wimleers.com/blog/jsonapi-1-eol.
On 7 January, 2020, the Drupal module JSON:API 1.x was officially marked unsupported. This date was chosen because it is exactly 1 year after the release of JSON:API 2.0, the version of JSON:API that was eventually committed to core. Since then, the JSON:API maintainers have been urging users to upgrade to the 2.x branch and then to switch to the Drupal core version.
We understand that there are still users remaining on the 1.x branch. We will maintain security coverage of the 8.x-1.x branch for 90 days. That is, on 6 April, 2020, all support for JSON:API, not in Drupal core, will end. Please upgrade your sites accordingly.