It has never been in force here in Denmark. A couple of them have had limits on visitors, and thus occasions where they couldn't let in more visitors, but even that has been quite rare.
Denmark has almost been the opposite of most other European countries when it comes to zoos (along with museums). The government has generally cared very little about them, not opening them until the very last phase of a given lockdown. When they closed for a month this winter, we were the only country in Europe to totally close zoos and museums as far as I remember.
But when they have been open, there have been surprisingly few (some may say delightfully few, others may say scarily few) restrictions compared to most of Europe. All indoor areas have been open, and as mentioned no pre-booking required and no real visitor number limits in outdoor areas. Masks in indoor areas were also only required for a few weeks during the spring of 2021 and this January, and have never been mandatory in outdoor areas.