Now that lists of accounts are stolen from websites regularly and published on the dark web I've wondered to what extent the victims need to worry about site memberships showing up in background checks.
I've never heard of an employer doing that and doubt any reputable employer would. In the U.S. I would think it could be a very narrow edge they put themselves on, one which could be very costly but not necessarily because of a lost job for a candidate. IANAL but I don't think any employment laws would prevent it directly (but some may indirectly). The fact that they are accessing a stolen asset however, means they could be sued over the use of the data pretty easily. Either way, any company with any ethics would avoid it as accessing the data itself is a quagmire.