This is unambiguously illegal and Facebook could stop doing this anytime it wants. Craigslist reminds its users about anti-discrimination laws before they post and provides a means for users to flag illegal ads.
> One reason you're seeing this ad is that Castore Sportswear wants to reach people who have visited their website or used one of their apps. This is based on customer information provided by Castore Sportswear.
> There may be other reasons you're seeing this ad, including that Castore Sportswear wants to reach people ages 18 and older who live in the United Kingdom. This is information based on your Facebook profile and where you've connected to the internet.
For example are actors/actresses, models, "entertainers"(hooters waitresses)
https://www.laborfinders.com/blog/is-it-ever-legal-to-only-h...
TBH it does seem a little discriminatory that a place gets to decide "all our waitresses are women", or "only women dance here", but I can see how industry's could be grandfathered in...