A source who has been exposed once does not quietly become a former source. They become the reason everyone else in that network stops talking to you.
Why journalism treats this as close to absolute
The Society of Professional Journalists' ethics guidance is direct on this, a promise of confidentiality gets kept, full stop, because the entire value of a source network depends on everyone in it believing they are safe inside it. There is a documented case of a reporter breaking that promise, the source lost his job over it and later sued the newspaper and won, and the damage did not stop there, other people with real information became unwilling to come forward once the story got around.
Why this generalizes past journalism
The same mechanics apply to any informal information network at work. Trading someone's confidence for a short-term advantage gets you that one piece of information. It also ends the second, more valuable piece that was never going to arrive through the formal channel anyway.
Where to start
Before repeating something told to you in confidence, even to someone you trust, ask whether the value of saying it now is worth what it costs the next time that person is deciding whether to tell you anything at all.
If there is any doubt, the default that protects you is silence, not a caveated version of the story.
Reading which confidences are actually yours to share in some limited way, versus which need to stay closed entirely, is a judgment only you can make about the specific person and situation. What this gives you is the cost to weigh before deciding either way.
There is a specific way to decline sharing something in confidence without it sounding like you are hiding something worse, and it is worth having ready before you are asked.