How to re-enable fixed volume controls on iPad Air?

1. Apple is not here. This is a user to user forum.
2. The Volume controls are in the same place they've always been since 2013. The Control Center. This has not changed in like 6 years now since they were put there.

Use and customize Control Center on iPad - Apple Support

Settings locations have changed over the years to accommodate new ones or better fit in with other options added, so yes, that is changed and will likely continue to change as settings evolve and more are added or others are removed or modified. Not much that can be done about that.

If you want to give Apple feedback, use their Feedback website.

click here ➜ Feedback - iPad - Apple

Dynamic volume controls were introduced with release of iPadOS 15.4 onwards. The “logic” behind dynamic volume controls is such that regardless of the screen orientation, the relative function remains constant. When understood and "the penny drops", they should make sense...

• When the physical buttons are positioned on a vertical edge of the iPad, the uppermost button will always increase the volume. Conversely, lower button will decrease the volume.

• When the physical buttons are positioned on a horizontal edge of then iPad, the button on the right will increase the volume - while the button on the left will decrease the volume.

For reasons known only to Apple, the ability to disable this feature is not available for any new models of iPad and iPad Pro that have since been launched - these models being:

• iPad Air5
• iPad mini6
• fourth generation iPad Pro 11”
• sixth generation iPad Pro 12.9”
• iPad10
• iPad Air M2
• iPad Pro M4
• iPad mini A17 Pro

This is documented within this recently updated Apple Support document:

Use the Home, side, and other buttons on your iPad - Apple Support

Apple invite submission of constructive comments and feature requests via its Product Feedback pages. If you would like to see the ability to disable dynamic volume control functions for your iPad, this would be the most appropriate channel through which to make your voice heard:

Feedback - iPad - Apple