Parsoid has its own extension API - see https://www.mediawiki.org/wiki/Parsoid/Extension_API.
In this first phase, we are targeting tag-hook extensions for migration.
The CategoryTree extension needs an update to work directly with Parsoid.
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Looking at the CategoryTree code, I think we can safely proxy this to the legacy parser for now without any impact. So, removing this as a blocker.
But, for anyone looking at this, once we do another round of updates to the ContentMetadataCollector interface (or even otherwise), it should be simple to create Parsoid-native handlers.