We are seeing reports of the Page Setup tab is missing the Other Pages section on the dialog.
We have confirmed this issue, and a fix is being released. If you need this functionality immediately, it is available in the Office Insider channel and is expected to be released to the Current Channel shortly.
Update: The fix has been rolled out to the Current Channel, build 16.0.7167.2040
How to switch from Deferred Channel to Current Channel for the Office 365 suite