Interesting. I agree the use of online ticket sales will increase dramatically. It had been very slowly gaining traction before but I think it will really take off now. The downside to requiring this is twofold. First it discourages spontaneous visitors (hey there's a sign for the zoo, why don't we stop in?). Second it discriminates against low income persons who do not have online access. The other question is how to handle members. Will they still be allowed to just show their card any time (I certainly hope so) or will they be required to pre-book their visit if numbers are limited? And if it is the latter and theoretically you could be denied entry because the slots are full then it defeats the purpose of buying a membership.
It will also be interesting to see how this affects exhibit design, particularly internationally. Here in the United States I have already seen a trend over the last decade for larger walkways and spaces, which was a response not to distancing per se but to accommodating larger crowds and specifically persons in wheelchairs. A great example of before and after this trend is San Diego Zoo. Before the trend they built Tiger River, whose walkway is uncomfortably narrow and steep (too steep for wheelchairs and honestly if it was designed today I don't think it would get approved by the local authorities). After the trend they built Africa Rocks, which has very wide pathways and was designed in a curving way to make a steep canyon navigable with a less steep walkway.