Based on my understanding, PLC is centralized primarily because there needs to be a global, authoritative source of truth for the current state of a given plc. You could in theory namespace a plc to a particular directory instance with a backwards reference or something, but I don't think it buys you anything when in theory you can just choose to trust a different PLC directory at the read/application layer if you really need.

At the end of the day, truly fully decentralized systems are literally impossible, there's always a centralized aspect (at least for bootstrapping) and it's usually DNS-shaped.

That being said, PLC directories are a problem that blockchains (yuck) actually solve very well: trustless, public ledgers. I would not be surprised if we see a separate implementation based on an architecture derived from such systems.