I can’t say why, but the init command should not have made any permanent changes, e.g., reboot should have gone back to what it had been for runlevel. On the other hand, init is becoming a deprecated way of working with this for Ubuntu (and some other distributions). Now you would probably execute this to go to non-graphical mode:
sudo systemctl isolate multi-user.target
(which is run level 3)
Or to go back to GUI mode:
sudo systemctl isolate graphical.target
This way some more modular (not always more popular) code would do the job rather than just an init script file. Should be the same result, but perhaps the older init scripts got lost in updates.