The rule against lying is GD / P&E specific. That has not changed.
In most other forums, while there is no specific prohibition of lying, many accusations of lying would be considered as an attack upon the user, which is forbidden outside of the Pit. This would be moderated on a case by case basis.
Saying something along the lines of “this is untrue” is probably going to be fine in most cases. Saying “please stick to the truth” is maybe a bit fuzzy as you are possibly insulting the user by implicitly calling him a liar. It might also be considered junior modding. Avoid things like “you’re a liar”. After all, it’s possible that the user is simply mistaken. It’s the old “attack the post, not the poster” rule.
As was previously suggested, saying that you did not say such a thing and asking for a cite would probably also work in most cases.