The FreeRTOS.org website remains the home of the FreeRTOS kernel, and AWS have simultaniously released the Amazon FreeRTOS operating system, which uses the FreeRTOS kernel, and adds connectivity and security libraries - all under the MIT license: https://aws.amazon.com/freertos
You do not need to be an AWS users to use the FreeRTOS kernel or Amazon FreeRTOS.
No - the license change is not retrospective. Old versions remain under
their original licenses. All old versions back to 4.[something] remain
available as tags in SVN and .zip file downloads in SourceForge.