Function sweep_entities_needing_specialization 

pub fn sweep_entities_needing_specialization<M>(
    entity_specialization_ticks: ResMut<'_, EntitySpecializationTicks>,
    removed_mesh_material_components: Extract<'_, '_, RemovedComponents<'_, '_, MeshMaterial3d<M>>>,
    specialized_material_pipeline_cache: ResMut<'_, SpecializedMaterialPipelineCache>,
    specialized_prepass_material_pipeline_cache: Option<ResMut<'_, SpecializedPrepassMaterialPipelineCache>>,
    specialized_shadow_material_pipeline_cache: Option<ResMut<'_, SpecializedShadowMaterialPipelineCache>>,
    render_material_instances: Res<'_, RenderMaterialInstances>,
    views: Query<'_, '_, &ExtractedView>,
)
where
    M: Material,

A system that runs after all instances of extract_entities_needs_specialization in order to delete specialization ticks for entities that are no longer renderable.

We delete entities from the EntitySpecializationTicks table after updating it with newly-discovered renderable entities in order to handle the case in which a single entity changes material types. If we naïvely removed entities from that table when their MeshMaterial3d<M> components were removed, and an entity changed material types, we might end up adding a new set of EntitySpecializationTickPair for the new material and then deleting it upon detecting the removed component for the old material. Deferring sweep_entities_needing_specialization to the end allows us to detect the case in which another material type updated the entity specialization ticks this frame and avoid deleting it if so.