The fact that they've been in every single Smash Bros. game. Ness & Jigglypuff both dodged the bullet in Brawl during the initial roster selection because of their OG status (as in, outside of the last-minute inclusion of Sonic that screwed Mewtwo out of Brawl).I see this written everywhere and while I agree with you for this iteration of smash, as time progresses Jigglypuff's relevancy to the franchise continues to wane. Ive never seen or heard any special rule regarding the original Smash 64 characters that they will never be cut.
Jigglypuff in particular has a new Fairy typing, giving her some relevance.