0.12 Migration: No mention of batch phase being removed

While looking into migrating kayak_ui I ran into batch_phase_system/BatchedPhaseItem being gone, but the migration doesn't seem to give any info on how to migrate things that depended on it. Presumably related to batching changes or the render schedule changes

Start by reviewing the 0.12 migration documentation and searching it for batch_phase_system and BatchedPhaseItem. Trace the related batching or render schedule changes to determine what users of the removed API must do; done means the migration guide explains the removal and provides a replacement path.

Written by the indexing model from the issue text.