Developers - dniym/IllegalStack GitHub Wiki
I get this is no where near a proper API, however I haven't had anyone express any interest in interfacing with illegalstack. This page will be expanded more as API features are added. Currently there are a couple NBT tags you can add to your items to have IllegalStack do things with them.
NoRepair -> Any item with this nbt tag will be blocked from repairs in the anvil.
NoEnchant -> Any item with this nbt tag will be blocked from further enchants on the enchanting table.
If you are a plugin developer looking to make your GUI that may break the vanilla stack limits, or enchantment limits then please consider creating a custom inventory object that extends InventoryHolder. Click here for code!