The tray indicator is passive - I can look at it when I want to, to estimate if / how long I should still leave it plugged in, and it serves the additional purpose of being able to quickly block screen locking and dpms. That's why, like @broulik, I have it always shown on all my devices, and I think it should be that way by default too.
Notifications and sounds however are active and distracting, and serve no other purpose. I ofc don't mind having such things as an option, but I think it should definitely stay opt in.