Square Enix all but ignored the 20th anniversary of Final Fantasy VIII, but you could argue that it was a different circumstance because they lost the original source code and couldn't port it to current-gen consoles as a way to celebrate the milestone.
In the case of Final Fantasy IX, the game has already been ported to the PS4, Xbox One, Switch, PC, iOS and Android. There isn't anything new that they could do.
Well they're currently doing a video series right now with the creative staff.
I also feel completely vindicated by the revelation from the staff that Jim Henson's Dark Crystal was used as one of the inspirations for this game.
Considering what little fanfare they give to FF anniversary's that are not FFI, VII, and X; I wouldn't expect much to be honest. We'll see since VI is celebrating it's 25th anniversary this year and VIII is celebrating it's 20th.
I feel like they're more inclined to celebrate the anniversary of Final Fantasy as a whole over any title. Though if you look at their various social media platforms they do give a shout out to individual titles once in a while.