Do you mean that new action button thingy above the volume controls? You can reassign to perform something else in the settings. Only a few options to choose from, but it's totally possible.

As for powering off, you can tap the ⏻ symbol in the upper right-hand side corner of the control center.

No they are talking about the new camera button on the same side as the power/siri button. Which is semi-ironic considering the volume buttons still work fine as camera buttons they just don’t also handle zoom (you can slide your finger on the button to adjust zoom). I honestly am more annoyed at the button than enjoy it, yet another button I accidentally press when I nearly drop my phone and now have the camera app open.

You can assign the action button to run a shortcut, which opens up thousands of possibilities for what it can do.