@John87, @Marit-Anne0 having Instant Book on or off does not affect your ability to read the reviews of a Guest. You can always read the reviews of a guest (if they have any).
Having Instant Book on does *add* the ability to see the Star Ratings of a guest. When that ability was added, nothing was taken away from the hosts without Instant Book.
If you cannot see any reviews for a guest, then the guest does not have any reviews (despite the age of the account).
You can test if you can read reviews (even with Instant Book off) by just clicking on the profiles of random people in these discussion rooms. You will be able to read any reviews they have, even if you have no connection to them at all.
The most recent guests having no reviews may just be a strange coincidence. There is no way to set up your account that hides reviews from you.