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