For presentation deck designers: Is it customary for potential employers to ask you to send them PPT files? I've ran into this recently and I don't feel comfortable sending other client's work to potential employers. I wouldn't send my ID files to anyone, and most of my freelance clients never needed to see source files, just the finished work.
I've never run into this before, but I did once interview with a firm who asked me to create a PPT on the fly for them during an interview. Maybe they want to see how you construct the work, how you set up animations, etc.
I personally have encountered lots of beautifully designed InDesign documents that were difficult, if not impossible to reuse. The designer didn't use styles, linked to images on their desktop and didn't package the file, etc.
In your situation, I would recreate an existing slide deck, but replace the text with placeholder text and images with generic images. That way you can send them a sample of your work, without compromising previous clients.