JAIF

Probably souls that have not become shinigami? It'd be awkward if, all of a sudden, captains just started vanishing in the middle of a battle for Soul Society's survival to be reincarnated. Plus, like I said, Genryuusai has been there for as long as Soul Society('s organized form). They wouldn't have known about it if it hadn't happened after Soul Society was created, and yet the oldest soul there has not yet been reincarnated. So it's either choice, it doesn't happen in a linear fashion or it only affects 'regular' souls. It's anyone's guess about the souls that are born in Soul Society.

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