What problem does this address?

Currently, there is an allow list of synced properties.

This prevents custom taxonomies from being synced. By allowing this list to be filtered, plugins can add additional properties.

For example, Co-Authors Plus uses a custom taxonomy to add additional post authors. Without this change, Co-Authors Plus would need to set redundant meta to participate in real-time collaboration.

What is your proposed solution?

Add a filter that allows adding (not removing) post properties to sync.