Migrating PHPUnit 6 to PHPUnit 9.1 in Magento 2.4
Magento 2.4.0 comes with a major upgrade of the PHPUnit framework, which is used for all types of tests: Static, Unit, Integration, API Functional, and the Magento Functional Testing Framework. Magento Architects decided to make an upgrade to the latest PHPUnit version, which is 9.1.

Decomposition of Magento Controllers
Magento 2.4 became a perfect opportunity to proceed with backward-incompatible changes that were waiting for years. When I was speaking at Magento Conferences about replacing inheritance with composition, I was not aware that I was going to be a part of this history. One such change was this Pull Request and its follow-up introduced by Vinai Kopp. Although this change was very expected, these PRs were not merged.