For reference, I've edited a very minimal FreeRTOS edition for the AVR architecture that can shim into the Arduino IDE as a normal library. It supports all of the classic Arduino boards.
I've written an overview of Arduino FreeRTOS on my site, covering how this was done, without compromising the "Arduino feeling".
The library can be found by searching for "Contributed" and "Timing" under the Arduino IDE Library Manager, or directly downloaded from Github Arduino_FreeRTOS_Library. The most recent release (as of writing) is v10.3.0 (LTS).
I hope that it is useful as a tool for getting hardware drivers going quickly under the Arduino IDE, and to remember just how light-weight FreeRTOS can be when needed.
It would be good if this (or something like this) can support people getting started with FreeRTOS.
The project and its subsidiary projects have been viewed over 8,000 times on the Arduino Create site.
Enjoy - Phillip