Function read_update 

async fn read_update(
    services: &Services,
    user_id: &UserId,
    fields: impl Iterator<Item = ProfileFieldName> + Send,
) -> Result<UserUpdate>

Reads back what the collected fields hold now.

The log records that a field changed and never what it changed to, so the current value is the one to send. A field the log names but the profile no longer holds is the removal a client needs to clear its own copy, which the proposal spells as a null value.