Hope they add IR learning for the Power button on the new Siri remote in tvOS 15.
For all owners of Apple TV with TVs, Projectors, AV Receivers and Soundbars without HDMI-CEC or incompatible HDMI-CEC, they need the new Power button to send IR for On and Off and IR for switching HDMI input to Apple TV.
Shouldn't be that hard for Apple to add learning IR for the Power button they are already doing the IR Learning for Volume + - button and Mute button. Settings App > Remotes and Devices > Volume Control. Select Learn New Device…
I realize that learning IR for Power On/Off requires you to cover up your TV/Projector IR sensor or have the Apple TV positioned so it can learn the IR for Power On and Off without the TV/Projector receiving the IR and turning off in the middle of learning.
To avoid the problem of a TV or projector turning off during Apple TV learning the Power On Off IR codes, Apple could do it with the help of an iPhone like they now do for Color Calibration since tvOS 14.5. Settings > Video and Audio > Calibration > Color Balance.
The new Power button should be able to control both a TV (or Projector) and Soundbar (or AV Receiver) via IR and switch the HDMI input to Apple TV. Some devices require discrete Power On IR and discrete Power Off IR, instead of Toggle Power On/Off IR. Apple needs to support Learning both types. (for example, to turn off many projectors you need to send discrete Power Off or send Toggle Power On/Off IR twice).
Apple advertises IR support for Power On/Off:
On the Apple TV 4K
Tech Specs page it states “Control your TV or receiver via IR or CEC”. With
footnote (3): "Powering on and off, switching input to Apple TV, controlling volume, and mute.