Canon EOS REBEL T7i Instruction Manual page 398

3 Custom Function Setting ItemsN
C.Fn-15
This is the setting for the lens retraction mechanism when a gear-
driven STM lens (such as EF40mm f/2.8 STM) is attached to the
camera. You can set it to retract the extended lens automatically when
the camera's power switch is set to <2>.
0: Enable
1: Disable
With auto power off, the lens will not retract regardless of the setting.

Before detaching the lens, make sure that it is retracted.

With setting 0, this function takes effect regardless of the lens's focus mode
switch setting (AF or MF).
398
Retract lens on power off