I first thought this up after re-reading Way of Kings again. There’s a scene where Wit is talking to Kaladin, and says, in reference to names, “I’ve many...I began life as a thought, a concept, words on a page. That was another thing I stole. Myself. Another time, I was named for a rock.” Nothing very telling about this, but it got me thinking, what if he was being literal about this. What if he wasn’t born human. This could explain both his longevity, but also his immunity to Shardblades.
It is my theory that Shardblades (dead ones) destroy Investiture, which is the power to manipulate souls in such a way that they influence the Physical Realm. A person’s Investiture is also their “soul”, and damaging/removing it has the effect of draining the person of life. This is evident on Nalthis where, giving up your Breath, the form that Investiture takes on that planet, leaves a person as a Drab, colorless, susceptible to disease, and lethargic. It is interesting to note that people who are injured by a Shardblade, but not killed, have the injured limb become pale, useless, and very much Drab-like. Based on this, I propose that Hoid does not have Investiture, therefore, making him immune to Shardblades.
If that’s the case, how does he have “access to all the magic systems,” as established by Words of Brandon? We’ve seen evidence of Lightweaving, Perfect Pitch, Allomancy, and so forth. I propose that he is not using Investiture to gain these effects. There may be another magic system, one that can mimic the abilities of Investiture, but is different enough to render one immune to it’s power, unless that power is used in a pure form.
This also answers such questions of how the 16 were able to destroy Adonalsium and why Odium was able to destroy other Sharholders without, himself, being destroyed. As seen with Ruin and Preservation, when one Shardholder tries to destroy another, they, themselves, are destroyed in the process. If, however, one had access to another power, one that didn’t feel the backlash that would come from attacking another Shard, it makes more sense.
Anyway, these are just some of my musings, as I contemplate the cosmere. Let me know if I'm totally wrong with this.