You didn't counter my argument about it being gimmicky. I feel it would generally be worse than enchanted diamond armor, but it would stand out at that one moment when you finally find a situation in which you don't take any of those damage types. Those damage types are also contrary to what players normally use, thus in PvP a player with bedrock armor would force others to change not just their strategies but their weapons entirely just to compete, but once the players had made these changes, they would win easily and the bedrock armor is thus pointless in PvP for anything other than ambushes.
If you're familiar with Pokémon, it's like Shedinja but with some extra HP. It's either underpowered or overpowered, with no real in-between.
Furthermore, you're trying to suggest that its rarity is a balancing factor, but in this case it's extreme rarity used to offset extreme unbalance. That arguably unbalances it even further, because it'll be so rare that solo you'll never find enough pieces to make it worth wearing, meanwhile in multiplayer most players will never see it but every once in a while someone will obtain a suit and use it against other players who aren't prepared for it because they don't expect it. For example, nobody is going to carry diamond arrows in their inventory on the off chance they meet up with someone wearing bedrock armor. The only way diamond arrows will come into the fight is if the one in bedrock armor attacks someone near their storage chest, and they run inside their house to go craft diamond arrows. In the time that takes, they might come back out to find their assailant waiting for them in diamond armor.
I don't think changing it to 99% fixes it. The problem is the way it is so strong against some types of damage and so weak against others. It effectively has correct ways to combat it. Balanced numbers might look more like 90% vs 70%. If it then was not enchantable but also immune to armor penetration, it would be good against large hits such as axes, bows, iron golems, endermen, or some boss attacks. In PvP a player would be at an advantage if they could use the right damage types against the bedrock armor but they could still win with basic melee or bow damage.
But that wouldn't fix it as it still has other major problems. Just that it negates enchantments and is instead stronger at the baseline without enchantments removes the power of choice that enchantments bring. It simplifies the armor and removes customizability.
And merely the fact that it's bedrock armor has potent implications for the "lore" or Minecraft. All in all I think you should stick to your guns and keep it at 100% damage reduction, but I think you should also give up on the idea of having this actually become implemented because I can assure you it won't happen. I think you should hang on to this suggestion for posterity or maintain it for its entertainment value, but if you want to suggest some serious armor, it should not be made out of bedrock.
Bedrock armor would still be useful in PvP. It's just that an entire team in Bedrock Armor would be an awful idea. If anything, think of it as being a bit like the Spy in Team Fortress 2: 1 in the team is useful. Half the team being Spies is awfully bad. In other words, it would be mostly useful if not used alone and by only a few players, specially as 1 of the best ways to counter it can be avoided by being able to obtain it (Namely, you need to reach the End, so fortifying the main island so other factions can't get there also avoids tiped arrows from being used against it).
Actually, even 1 or 2 pieces would be worth wearing, due to the high durability, and that an incomplete set also has a lower speed penalty.
Actually, even with 100% resistance, a sword would still be enough. In fact, punching would be better, as in that case, the idea would be to break the armor, or at least the weakest part. Not being able to enchant the armor at least makes this a bit more predictable (Plus, it already has a high damage resistance to almost anything, except for the damage types it is supposed to be weak to. Enchantments would, except for enchantments such as Unbreaking, Curse of Vanishing in PvP, and Mending, which would make it too difficult to take out, would be redundant).
As for it having strong implications, the only thing it really indicates is that there's a way to mine and work with bedrock that isn't available to the Player, or there used to be. It seems strong, but if anything, it adds to more theories. For example, in the Adventure Update pre-releases, Endermen COULD pick up Bedrock, so it could act as a nod to that, even if it was probably unintentional.
I think this idea is horrible and I am concerned that it is getting a lot of support merely because the OP is well thought out, detailed, and easy to understand, as if these are merits so powerful they can salvage the base idea no matter how bad it is.
It ignores the existing armor penetration system and instead goes for a gimmicky system of being immune to some types of damage while being vulnerable to others. I don't think I need to explain how incredibly unbalanced this is.
I consider this suggestion to be an excellent example of everything that is wrong with the Minecraft suggestions critics community.
And armor penetration is not effective enough when most in-game mobs don't deal enough damage for it to be really important, while being inmune to SOME damage types makes it just plain more useful, specially as it's more predictable. Plus, you're more or less assuming that it's easy to find. It's actually more or less at the same difficulty as Elytra, and may be even rarer, as there's 1 Elytra per End City, but each End Bunker doesn't even have a guarranteed chance to have a Bedrock piece. Finally, the fact that it doesn't protect against all damage types prevents it from being OP, specially as several of them are rather common. In fact, Bedrock Armor is easily worse than Diamond armor in the Nether, as it would only be useful against Zombie Pigmen and Magma Cubes, while running away from Wither Skeletons, and dodging both Ghast Fireballs and Blaze Fire Charges would be more difficult...And those 3 mobs can damage a player with Bedrock armor.
Bedrock Armor should have a finite durability (Between Iron and Diamond armor), and the slowness penalty should be changed so it's a more general debuff, with a lower speed penalty that is paired with an attack speed and mining speed penalty, and increased hunger loss while sprinting.