So, what do know about the differences between old Shard Plate and new Shard Plate?
Radiant Era plate:
Glowed while in use
Did not interfere with surgebinding
Helmets could be dismissed / summoned in an unspecified, but short, time frame
Gemstones are neither confirmed nor refuted as being inside them
Current Era Plate:
Does not glow
Potentially interferes with surgebinding (possibly only if the binding is granted via Honorblade)
Must be put on the same way as traditional armor
Gemstones inside give some sort of power-armor like behaviors
Additionally, we know that the gemstones in the hilt of Shard Blades were added post-Knights Radiant, so that you could sync your heartbeat with the dead spren inside the blade, and summon it. From this, we can surmise that the current state of Plate may not be the same state that plate was back in the KR era.
As far as what I think - I would not be surprised if Shardplate were a fabrial. My reasoning for this is as follows:
When the KR threw down their swords / armor, and there was a crazy melee to get pieces of things, there is absolutely no way anyone could have put on the plate by themselves, and whatever allowed the KR to summon/dismiss pieces of their armor would be just as dead as the spren in their swords. So, people would likely have been left with mismatched pieces of armor that they grabbed. Then, later, as they fed the gemstones stormlight, the armor would grow the rest of itself until it was all one set again. This would mean the gemstones are there as a way for the KR to store stormlight nearby, but without having to infuse themselves and have it slowly leak out, as well as a way to fuel the armor's "more than just armor" abilities.
I'm also not going to be surprised if it turns out Shardplate is some sort of "body of your bonded spren" type deal. Reasoning:
Shardplate seems to be a living thing. You can grow new plate by feeding it stormlight. Stormlight seems to repair damage based on how the person (or thing?) thinks/feels about themselves (cognitive realm?). I think if you say all 5 ideals, your spren can become armor, should you need it.
Or, maybe Shardplate is some sort of symbiotic fabrial/spren bond. Built by men, inhabited by Spren, used to enhance surge binders? I think the fact that the heralds seem to not have plate would suggest that it's man-made or spren-based, rather than Honor granted.
Those are just my rambling thoughts.
