Live Preview is one of those important features that are not available yet on PowerPoint 365 for Mac.
If you want to ask Microsoft to add this feature to PowerPoint 365 for Mac, you can visit the PowerPoint UserVoice site, and vote for this suggestion. Someone has already put up this suggestion, so do vote for this feature. Suggestions with more votes are implemented sooner than others.
You can learn how the PowerPoint product team looks at UserVoice suggestions in this feature with Shawn Villaron, Partner Group Program Manager at Microsoft: PowerPoint Feedback and UserVoice: Does This Go Anywhere?
Figure 1: Live preview in PowerPoint
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