Then to answer your Q: a good sharding approach should let you see all digitally signed claims (including when those claims were made) with probability --> 1.0. I'm framing this probabilistically because many sharding approaches rely on that definition. (And even non-sharded blockchains like Bitcoin itself.)

So, what you are describing seems like a way to accomplish behavior of this kind of data structure with a different one so you may get bettee scaling out of it.