Source: Microsoft
As with many experiences on mobile these days, Microsoft is bringing the option to create slides in portrait mode when you're using the PowerPoint mobile app. The feature comes to the public beta of the Microsoft Office Insider program (version 2.68 or later) for iPhone and iPad devices.
Microsoft pointed out and acknowledged the "shift towards mobile-first content creation." And it now gives you the flexibility to create presentations in portrait or landscape. The option to change the orientation is accessible through the Design option.