i think to be a plugin it needs to be completely decoupled which is tricky to do in a game,
for instance i have an inventory component which is mostly self contained, but has a Max Weight which is determine by Strength which is on an Attributes component.
so as hard as you try to Decouple things they end up referencing each other.
I put helper things in plugins, kinda like a function library i might reuse but ultimately its whatever servers your game best