I'm sorry that you had this experience.

I'm always (constructively) honest in exit interviews, and have even been told that companies have changed things in response to things I've said (taking my feedback and similar feedback from others).

I'm wondering about the legality of a company threatening you over Glassdoor reviews, especially given the difference between how "libel" works in the UK (you have to prove it's true) and in the US (you have to prove it's not true). That said, I understand why you took it down.

We do have a duty towards others, and there was an interview where I heeded the red flags and withdrew my application on the spot after calling a premature end to the interview.

My colleague told me that she'd accepted a job there and I took her aside and warned her about all the red flags I'd noticed, and she took them on board but started the job anyway.

I started a job at our sister-company and, some months later, she started there, too.

She told me that I'd been right about everything, and thanked me for the heads up - she'd gone in with her eyes wide open - but sticking it out for a few months gave her a title boost and pay bump that meant she could gain a promotion at our new company.

There's rarely, if ever, any downside to being constructively honest, and most of the time, they'll thank you. That said, it really doesn't hurt to caveat your GlassDoor review with "I felt" and "I got the impression" rather than making any concrete claims about how it is.