A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Techteacher123,
From your description, it seems that you define the customized slide master via the steps in the article, save the PowerPoint file as a template and upload it to a site library in SharePoint Online. If yes, the customized slide master should be seen in PowerPoint Online when clicking on New Slide. I suggest you scroll down the bar to see if you can find the slide master. Meanwhile, I suggest you open the file in the PowerPoint application from PowerPoint Online, re-add the slide master and open the file in PowerPoint Online to see the result again.
If you create a new template, what’s the result?
Best Regards,
Cliff