The inconsistency mentioned regarding the new customizable tab bar feature in the developer beta 1 of iPadOS 18 is that some apps, like the Files app, have a sidebar but do not yet have a customizable tab bar.
To add menu options to the new tab bar in iPadOS 18, follow these steps:
Now, the tab bar will provide quick access to the areas of the app that you deem most important. Keep in mind that certain items cannot be removed from the tab bar if they're considered core to the app's UI, and not every app with the new tab bar supports customization.