Auto-replenishment puts cancelled rooms back up for sale on Booking.com. This is a default function which cannot be deactivated.
Aside from that, there is also a ‘Reopen closed rooms’ setting, which is also active by default – but which can be deactivated on request. Please see this article for further details on this setting.
For more information about how auto-replenishment works, have a look at our introduction to the topic.