It’s in use at my company after being implemented after the Germanwings crash and I personally have mixed feelings about the policy.
Mental health challenges for pilots are as much taboo now than they were five years ago, in the run up to the horrific Germanwings accident. Whilst some regulators and airlines have developed their support mechanisms for pilots with mental health challenges, many have not.
Having the door opened and closed on more occasions is not ideal but the risk this poses will vary greatly, depending on what type you fly. Pax can’t easily see the flight deck door on the type I fly, so this is far less of a risk than it may be for some others.
Our cabin crew are trained to not initiate conversation on the flight deck unless done so first by the remaining pilot. It seems to work well for managing any distractions.