I teach high school students from time to time and over the years I have accumulated a large library of STEM training material. Of which there are a lot of PowerPoint slides with interactive Flash content embedded.

When MS and Adobe turned off Flash for Windows, the MS Office flash plugins also stopped working. I had to set up a VM running windows 7 to be able to use them again. It is not as smooth as running powerpoint natively but it's good enough for classroom demonstrations.

However when students ask me to share the slides I have to tell them that while they are welcome to have it, about half of the stuff won't work properly because of a rich media plugin they are too young to have heard about. Besides, I don't want to teach them how to set up their own VM, and I doubt they are willing to go that far.

I have spent some time looking into this problem and have not found any third party tools that will allow embedded flash to work again. So much for the digital dark age.

I'm curious if you've tried ruffle: https://github.com/ruffle-rs/ruffle

It will not work for exactly your problem (powerpoint plugin vs html5 webpage), but I've found most relatively simple flash programs run under it. If the embedded flash content you've got is relatively simple, you might be able to run it in a browser with ruffle.

At which point you could swap out the powerpoint presentation from embedding flash alone, to embedding flash + having a link to a webpage for modern powerpoint users, which wouldn't make for good presentations, but would be better than nothing for students reviewing it.