In Microsoft PowerPoint, you have Slide Masters so you do not have to do the same thing on every slide but in PowerPoint online, you have to do the same thing on every slide and it just wastes time when you do that so there should be Slide Master on Online PowerPoint and not just on PowerPoint.
Eric S. commented
I am surprised that there is no way to edit Master Slides in PowerPoint Online. Google Slides has been able to do this for years.
My Office 365 subscription should have online apps with more feature parity to their desktop versions.
Can you allow master slide editing for teachers to create online interactive sheets for students
Put master pages in simplified ribbon
Where is slide master Option in View
Roger Heckly commented
Tags can help to save data in a presentation, on a slide or for a shape. This makes it also easy to identify a programmatically created shape / slide etc.
Today there is no way to save information on a master layout via tags as possible on presentation or slide / shape Level.
I like to save already in the master some information via tags - on Master, Layout and Shape Level.
This would make it easy later to programmatically identify the slides in a presentation, objects on a slide when the slide is created based on a specific layout.