Isn't it common knowledge that people in new jobs make more money?

Searching any listserv or salary database will get you that information.

Not sure that's true. In most of the country/world local employer don't have salaries in a publicly available database.

And there may not be enough of a tech scene in a city or region to bootstrap such a database easily.(assuming we're concerned specifically with tech/software salaries)

In Orlando we have a Slack group for all the devs in the city and at the end of the year they do a salary / tech survey everyone fills out. Gives anybody in the city a good idea of what compensation looks like. Other cities could build similar surveys if they want to. You dont need a million devs just enough to be willing to pseudo-anonymously answer. Then based on pay by years of experience you can gague how low your salary is.

In the US, the Bureau of Labor Statistics can give a rough histogram of salaries by occupation for most metro areas. I can't ever remember how to find the data though.

It would get you averages or the salaries of people whose marketability you don't know.

It doesn't get you "Hey! That bozo now earns £20K more than me"