I’m not sure what you mean by this. We should already be exporting the same texture format as the original loaded texture format.
I believe @brightMoon is referencing this issue: GLB export changes original image which has been added to our backlog 
If so, should this not be marked as breaking change, too?
Hmm, I don’t know if this qualifies. The issue that was solved by #13909 is still solved by #15869. No public API contract has been changed, and I’d consider the details of an exported glTF file to be internal. But it is true that, if someone were interfacing directly with the exported glTF file, the numbers will be different.
At least, if #15869 is a breaking change, then that implies #13909 was also a breaking change.