I worked for a company that kept one job posting open for more than 4 years. They've used it to hire more than 100 people, but unless you worked there you wouldn't know.

Not too surprising. I've been at companies where a job posting is hyper-specific to one job description, as soon as I (as a hiring manager) change a single word, HR makes a new job posting and cancels the old one.

And also I've been in places like you describe, where one generic "Software Engineer" job posting is reused to hire many people into many teams, since they're all "Software Engineers" so they just recycle the same posting for everyone.

It varies a lot depending on HR culture.

Interesting to hear this data point because everyone would just claim it was a sham job that some companies post to get a feel of the market.