Missing Fixed Position Volume Controls

The setting should be in Settings > Sounds

1. Go to Settings > Sounds.
2. Turn on Fixed Position Volume Controls if you want the volume buttons to always have the same behavior. Turn it off if you want the volume buttons to change dynamically depending on the orientation of your iPad.

Use the Home, side, and other buttons on your iPad

From the above-linked Help Topic:

These iPad models feature always-on, dynamic volume buttons: 

• iPad Pro 11-inch (4th generation)
• iPad Pro 12.9-inch (6th generation)
• iPad (10th generation)
• iPad mini (6th generation)
• iPad Air (5th generation)

For some reason, it seems Apple removed (if you believe it was there in the past) the option on the above iPad models.

Later on when I get access to my iPad mini 6, I will check if the setting is there on it since I have personally not yet updated it to iPadOS 16.4.

You can leave feedback to Apple regarding this change:

Feedback - iPad - Apple

Dynamic volume controls were introduced with release of iPadOS 15.4 onwards. For reasons known only to Apple, the ability to disable this feature in favour of fixed volume controls 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

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

Were you running a previous version of iPadOS 16 but below 16.4 previously or did you update from iPadOS 15.x up to 16.4? If you were on iPadOS 15.x and then recently updated directly to iPadOS 16.4 (skipping other versions of 16), this is likely why you have seen/noticed the change as I believe it was changed/removed beginning with iPadOS 15.4.1.

Regardless, send Apple feedback at the link in my other post.