[ojame]

In many countries, including the UK (where the author is from), it's not illegal to talk openly and publicly about your salary. Contracts can have 'do not disclose your salary' but, in places like the UK, this is legally unenforceable[0]. It gets pretty grey though, in the UK it's allowed to protect you from underpayment and from bargaining etc. I reckon disclosing it publicly can be seen as a form of a basis for collective knowledge at the organisations, though.

0: https://metro.co.uk/2018/11/09/can-your-boss-stop-you-talkin...

[nonethewiser]

Nor is it illegal in the US. Whether or not it's illegal has no bearing on of the employer will be upset.