My company had a similar spreadsheet internally that was self-selected and it was incredibly useful. I found that I had come in at the low range for my pay band and got a $10K raise in the next annual salary review.

It's anonymous. I'm not sure why you think it'd be so susceptible to bragging or complaining. A lot of people, me included, are data nerds, and will contribute to the dataset regardless of our personal situation simply because we want the data to be as accurate as possible.