RHEL 8 will use Python 3.6 by default. Debian is planing to use Python 3.x as the default in the next stable release (slated for early 2019). Arch Linux and Fedora both use Python 3 as default. Ubuntu is not as clear (can't find definite info), but I think 18.04 uses Python 3 as default, at the very least 20.04 will have to use Python 3 as default because python2.7 is EOL'd in 2020: https://pythonclock.org/
RHEL 8 will use Python 3.6 by default. Debian is planing to use Python 3.x as the default in the next stable release (slated for early 2019). Arch Linux and Fedora both use Python 3 as default. Ubuntu is not as clear (can't find definite info), but I think 18.04 uses Python 3 as default, at the very least 20.04 will have to use Python 3 as default because python2.7 is EOL'd in 2020: https://pythonclock.org/