Side skirts are supposedly only for the purpose of stopping CE projectiles such as those from RPGs (e.g. PG-7VR). Not going to do much against KE threats, but if an APFSDS is fired at the side, it is going to go through with or without ERA or composite modules.
Supposedly, the reason why these additional side armor is not added is because it would be pointless to have it during peacetime training. Tbh that doesn't make since because even if that were the reason, at least we would see even just one instance of it being applied, such as the way we have for the side armor of ZTQ-15.
Common reasoning would be that if the PLA had to hypothetically deal with a Grozny-type situation and these tanks had to be stupidly charged in, then at minimum they would have the side hull reinforced with light ERA at least. More probably than not, the 99/99A already has such kits available. But the fact is that there is no visual confirmation of its existence yet, and so it would still be more reasonable to believe that it does not have one ready at least for now.