Series 9 doesn’t exist. In the Red Dwarf Special, they make references to Series 9, calling it one of the best series ever, amazing, incredible, and so on. Story-wise, skipping past Series 9 allowed them to change the setting and characters of the show, without actually resolving the cliffhanger from Series 8. Maybe they’ve just decided to skip it altogether, or maybe they’re building up to doing it retroactively.
In case it wasn’t obvious, I think Series 7 and 8 are worth watching. There’s some clever stuff in there, and any Red Dwarf is better than no Red Dwarf.