The thing is, that it is only required if the company is located in Germany. Thats why pornhub, a company run by Germans (afaik) does not have to do it… It was in the news multiple times, when the government tried to remove that condition so all porn sites accessed from Germany have to verify the age of the German visitors.
TL;DR Most people are against a age verification where you would input your personal ID or credit card. Adding a popup would be nice though (if it doesn’t already exist)
Thinking about all the included responsibilities, legislation, etc. gives me headache so I will not bother setting up a NSFW instance, from which thought the question originated… Thats propably why there isn’t a good one right now