Dear David Lindsey1,
Thank you for sharing your question with the Microsoft community.
I understand that you are experiencing some issues with the ‘Reuse Slides’ feature in PowerPoint, I understand your frustration and will try to help you troubleshoot.
I also tried to test it in Microsoft 365, but I seem to have a different situation than you, according to the official article, this feature should still be supported:

In order to better understand the status of the issue, I would like to confirm some information with you first:
- What version of Office are you currently using? You can click File->Account->Share the screenshot of the version information on the right side with us. (Please be careful to cover your personal information*)*

Looking forward to your further updates and hopefully we can gather more information to troubleshoot if this is a new issue that happened recently. We will also continue to gather information from the community to see if any users are having similar issues.
Thank you for your understanding.
Best regards
Isabella-MSFT | Microsoft Community Support Specialist