Figma Design allow you to organize in a sub-folder like structure inside files, with multiple pages possible per project.
Figma Slides however only consists of one top level presentation, not allowing you to create multiple presentations inside each file. This leads to a lack of organizing possibilities, and if you have multiple presentations or projects per client, things can get chaotic pretty fast.
The option to nest folders inside each project in Figma Slides is a must, as I see it, or it will be impossible to keep track of presentations in the long run.