On Galaxy S20/S20+/S20 Ultra, the default settings for side key are as follows:
1. To power on: When the device is off, press and hold the side key.
2. To power off: (1) Select power off icon in the quick panel (2) Press and hold the Side key + Volume down key.
3. Screen capture: Press side key + Volume down key
As a user you can remap the key feature for power off menu by pressing and holding the Side key like previous devices.
Launch the Setting app and tap Advanced features > Side key, and then tap Power off menu under Press and hold section.