The reveal.js file format is HTML, an open standard, and will still be readable decades from now.
Proprietary apps like Google Slides, I have to continually assess whether their file format is closed or has recently switched to being closed — because it is in the interest of the company to lock in its users and exploit network effects.
That's true, although to some degree the separation of semantics from styling in HTML/CSS/JS means that the structure of the document will be preserved. And at least HTML is a text format! How much can you recover from a proprietary binary format, when the app's not around any more? At best somebody has to reverse engineer it, and it could change in all sorts of undocumented ways between versions of the app.
I’m a tech school teacher and we version control all our Reveal.js slides in Markdown, sharing styles and certain images.