I will voice my opinion on this. Imo this is a very bad decision to put it lightly. S4 was meant as a “so you have something to do til Dragonflight that might also be a tad fun”. So baring the LFRaiders from this seems be done without any proper reason. And while im usually not a big fan of all the “fomo posts” i will agree that this applies in this case here given the popularity of the slime cat mount to begin with.
That we also assumed it would be obtainable through any difficulty with LFR also being fated as well. Which heavily adds to the frustation of the playerbase right now.