Function read_from_app_data_dict 

pub fn read_from_app_data_dict(
    id: ComponentId,
    mls_group: &MlsGroup,
) -> Option<Vec<u8>>

Look up the component’s bytes in the OpenMLS AppData dictionary.

The commit validator in xmtp_mls pulls the pre-commit stored bytes for a component and threads them into expand_app_data_update_to_changes as old_value — the validator uses that to resolve RemoveByHash mutations back to their underlying inbox id. xmtp_mls’s app_data module also uses it from process_message_with_app_data, stage_app_data_propose_and_commit, and pending_app_data_updates.