This is not a dark pattern any more than giving a company like GlassDoor this information is a dark deed.
Honesty would mean telling the user upfront that the info they are looking for will be available after the set up an account and leave a review of their current employer.
When I looked up an employer I’d joined recently, I gave a review as someone who had just started. Their most recent negative review was quite negative and easily traceable to the person who had most recently quit and was on their last days.
So reviews are either dishonest or not anonymous when they have to be recent; this pattern appears to incentivise some degree of pollution or self-incrimination.
The review I gave before leaving was much more informed and honest, in part because I timed it and in part because there were several people with my profile leaving at the same time.
But whether it's a dark pattern is not what makes this particular instance egregious in my mind. I find it egregious as it places me in a situation where I have to either lie by providing false information and misusing the platform, or make an uninformed decision I may not otherwise have made to consequences that could potentially have been avoided were the decision in fact informed.
Consider a workplace culture rife with harassment. That's a harmful situation to enter. Reading reviews of an employers workplace culture on a platform like GlassDoor isn't going to make it impossible to encounter such environments, but it will make it easier to avoid them. But I'm forbidden from doing so because I cannot trade for it. As I indicated in my original comment, the implication here is that I'm only entitled to seek safe work environments if I can afford to pay for the privilege.
Per their own About page:
> Every day, we’re inspired by a vision to make positive workplace change through radical transparency. Through the products we make and the communities we create, we’re breaking down barriers that lead to discrimination, pay gaps, and toxic work environments.
This tactic of demanding upfront payment (in the form of a review) is antithetical to their stated goal and philosophy. As a user I think my other complaints are fair, but my offense, as you say, comes from this.