Current situation
The agent system service for the AWS Session Manager is not installed by default. It can be started as a Docker service with a customized image.

Impact
Users don't have this functionality available unless they know the system very well and are ready to run the agent themselves.

Ideal future situation
The agent runs by default if there is no drawback.

Implementation options
Run it via Docker with a customized image as written here. There is an OEM package for EC2 but I recommend to include the service file not in the OEM package but in the regular /usr partition and only start it under the systemd unit condition that the kernel command line includes the OEM ID, i.e., KernelCommandLine=flatcar.oem.id=ec2 (and maybe add KernelCommandLine=coreos.oem.id=ec2).
Build the Docker image in our quay repo and tag it with a version.

Additional information
All relevant links are in the blog post linked above.