Hi Hjord, welcome to kiwitrees. Thanks for the reminder about the support link. I will get that fixed.
In webtrees there is the “Require visitor authentication” option.
I can’t seem to find it in kiwitrees?
I’m afraid you won’t find it in kiwitrees. I removed it a couple of versions ago. It was always more trouble than it was worth to maintain, and rarely used except by users who do not realise the many alternative settings that more than adequately protect privacy.
If you really want a web site that is not accessible by casual visitors it is better (IMHO) to use a private server, or protect the entire system with server-based password controls which your web-host should help you create.
It is not a feature I will be adding back at any time.