The storyline release notes and discussions about autofit seem to indicate that it's not just the textboxes that are updated but the entire project itself is set this way. I believe this means that you wouldn't be able to delete those slides and in my version of 360 there isn't a way to convert back to the non-autofit version of a project once you've converted it over.
In the notes about autofit storyline seems to know this and it mentions that a backup copy should be saved if it's needed in previous versions.
https://articulate.com/support/article/Storyline-360-Text-Autofit-Improvements#:~:text=in%20case%20you%20need%20to%20access%20it%20in%20Storyline%203
https://articulate.com/support/article/Compatibility-Between-Storyline-360-and-Storyline-3-Project-Files#:~:text=feature%20won%E2%80%99t%20open%20in-,Storyline%203,-or%20earlier%20versions%20of
