From what I've experienced, having all of the protection types reduces the overall damage that you are protected from. It appears that when struck by an iron golem, that far less damage is reduced when I adorn armor that has Fire protection 4, protection 4, projectile protection 4, and blast protection 4. If I wear armor that only has protection 4, then it seems to be more effective.
Comments 2
that's not true. The point that i'm making is that the more enchantment varieties that you have, the less effective ones armor appears to be over all. I never stated anything about there being a damage reduction cap. ilmango focuses on how having all of the protection types simply caps out at a certain point, and doesn't mention armor having reduced effectiveness due to "over-enchanting."
If you have all the protection pieces on every piece of armour, then this would be intended.
ilmango explains it well during his video: https://www.youtube.com/watch?v=pIBIuSTdQD8