Anytime you add an Xcode 16-only feature to your project, the format will automatically be updated.

For those who ended up on this thread because of a similar issue with the Project Navigator, explanation below:

When you right-click on the Project Navigator in Xcode, you have the options "New Folder" (Xcode 16 only) or "New Group" (any Xcode version). When you create a New Folder, your objectVersion will automatically be bumped to 70.

• Right-click on any Folders you've created and select "Convert to Group".
• When you've finished converting all folders to groups, Xcode won't automatically set the objectVersion to 70.
• Verify that your project settings are still set to Xcode xx (9.3 or 12.0 or whatever you wanted).