What about former employees? At least in the Glassdoor use case, they need to support feedback from former employees in addition to current employees.

And Blind only verifies your employee email on signup, there's no renewal of that verification - you can leave your employer and still retain your original access.

Email verification is the simplest, employer email addresses are generally considered public info, and it also helps detect/deter bots. OTOH, it goes through the employer's email system, so your boss could know that you just signed up for Glassdoor/Blind/etc.