There should be 3 variants so the user can decide what types of shields the enchantment can go on.

Can simply be done by copying the current enchantment class twice and changing isAcceptableItem

There isn't really a reason to. If someone needs it to only be applied to certain shields they can just extend the class.

• If someone is making an enchantment that they only want their shields to have they probably wouldn't want it to be on other modded shields. (ex. they make a shield with spikes & the enchantment makes the spikes freeze the enemies)

• why would an enchantment ever need to differentiate between shields that support banners and shields that don't? Unless it's something extremely specific that changes how banners are displayed on the shield. In that case, they can just extend the class since what they want is probably very specific.

• The base class by itself doesn't really do anything special, it's not used in any mixins. It needs events for it to actually do anything.

I just think that if someone has needs that specific they can just extend the class and override isAcceptableItem

Yeah true