The Drupal hosting service will start blocking by default any project in the pipeline that uses PHP 7.3 or lower version as of 15 December 2021 to raise awareness of the need to update to PHP 7.4 or newer. This will be a soft block with the possibility to bypass by inserting a commit message.
As of 15 February 2022, there will be no possibility to bypass anymore. Therefore, projects need to update to PHP 7.4 in advance to continue deploying in the Drupal pipeline after that date.