This is optional. The first time you activate Assistant on the Pixel 8a (such as by holding down the power button), you’ll be given the opportunity to opt-in to replace Assistant with Gemini.

Google confirmed this in an issue tracker thread: “The Battery Health Indicator feature is currently supported in Beta 3 on Pixel 8a and Pixel 9 products including 9 Pro Fold. Due to product ...

For the Android 16 Beta, only Pixel 6/Pro, Pixel 6a, Pixel 7/Pro, Pixel 7a, Pixel 8/Pro, Pixel 8a ... fully power down your handset. Then hold down on the Power button and the Volume down button ...