It’s good that ROS2 has stardardized on python3.
As for ROS[1], support for pyrhon2.7 vs python3 comes down to the package author/owner.
Some package support python3 while sone doesn’t. It’s completely possible to use ROS with python3.
I don’t know if a guaranteed way to correctly notify users if their collection of code does/doesn’t work with a specific version of python.