microROS puts ROS2 onto microcontrollers.
The major changes compared to "regular" ROS2 is that microROS uses a Real-Time Operating System (RTOS) instead of Linux, and DDS for eXtremely Resource Constrained Environments (DDS-XRCE) instead of classical DDS. Above that, we run the ROS2 stack! Well, with a few cool improvements for taking advantage of micro-controller specific things, but largely the same.
Source code can be found at https://github.com/microROS/ and we answer both tickets and pull requests as usual. If you have questions on concepts and development, you're very welcome to post to the embedded catgory on ROS Discourse. For usage questions, head over to ROS Answers and please tag your question with embedded.