Yes, I am convinced this is the new 'normal' - the only real variation being now whether the on-line system 'insists' on, or just 'advises' pre-booking.
This probably is a best-of-both-worlds scenario, as it gives those wanting the assurance of a slot the comfort of purchasing a ticket up-front, giving the zoo an idea of potential foot-fall - whilst still allowing a casual decision for those who want to decide at the last minute, usually made as a result of the weather forecasts.
I just wish they'd remove the 'essential ' bit off their websites if the reality is it not the case anymore. Everyone knows that pre-booking normally brings a discount and saves(?) time on entry- or does it?