Why is Launch darkly.com in my website data

https://launchdarkly.com/how-it-works/ but you are probably already aware of that. It is most likely incorporated as Javascript code incorporated in one or more of the websites you loaded within the past few days.

I don't believe there is any reason for concern. Safari already incorporates robust privacy protections so if you're worried about websites harvesting your personal data (for example) Apple makes that very difficult to accomplish.

If I had to guess from the description, it's a website developer tool intended for them to determine how users interact with their services, and possibly improve upon or optimize whatever reasons they may have for deploying those websites and services associated with them. If you perceive you are being exploited as a kind of test subject, you're not wrong, but that's how a lot of massively deployed services work these days.