I have been using block displays as client-side root entity for my holder + sub-elements (item displays)
for my living entity subclasses (with attributes, goals, etc)
In short, BLOCK_DISPLAY as return value for getPolymerEntityType

Now with this change:

The server crashes, since vanillaHolder is null.
Would it be possible to either check for a LivingEntity or DefaultAttributeRegistry.hasDefinitionFor ?

Or is there a deeper reason for the changes that I am not seeing yet?

I'm also getting out of bounds errors in polymer$createEntries when doing something like this (Mojang mappings):

    public static AttributeSupplier.Builder createAttributes() {
        return Animal.createMobAttributes()
                .add(Attributes.MAX_HEALTH, 6.0)
                .add(Attributes.MOVEMENT_SPEED, 0.25);
    }

    @Override
    public EntityType<?> getPolymerEntityType(ServerPlayer player) {
        return EntityType.RABBIT;
    }

The entity and its attributes are registered properly

Ok, issue is simple bug with boolean not being inverted at https://github.com/Patbox/polymer/blob/dev/1.20.5/polymer-core/src/main/java/eu/pb4/polymer/core/impl/networking/PacketPatcher.java#L99, will fix that tomorrow