This cuts both ways, usually the agreements are bi-directional so they may not disparage you either. And honestly this has been super common at the executive ranks for decades, like many of the other agreements that have filtered down to the average employees.
Outside of this though, there are always companies who abuse employees and try to hide it, this is not new nor is it going to magically disappear anytime soon. There are good companies that use these techniques to protect themselves and their employees rightfully. This is because there are a large group of people that are quick to lash out and embellish stories about a company which can be highly damaging, affecting dozens or even hundreds of lives. I am sure you have worked with at least a few of those types of people. It of course cuts the other way, where the company is a total loser and abuses their employees and tries to use this and other techniques to hide their bad behavior. And that is just as horrid and immoral.
In the end, just as your friend did, most times the news gets out about the abusers and people will get the message not to go there which will affect the company and in the end it can be terminal over time. Usually these companies get caught up in litigation eventually that either does them in or winds up with the effect of cleaning out their executive ranks.
As for should you take a job at a place that uses them, that is a personal choice and you should review the company and ask around in non public ways (e.g. not the internet). People are people, and information will always get out. In my opinion, it is not rare for senior positions to have these agreements so I am totally comfortable with them, assuming they are bi-directional. Uni-directional is out of the question and would be a massive red flag.
Also remember, that none of these agreements covers illegal behavior, so if you saw them doing something illegal like dumping toxic waste and reported it or quit then reported it, they'd have little recourse against you. It doesn't mean they couldn't sue you, but they wouldn't get anywhere as long as you were accurate. Same goes for if you were stealing from the company and they found out after you quit, they could turn you over to the authorities and the agreement wouldn't protect you. Again you could sue them, but the outcome would be you'd lose.