If I'm interviewing a candidate, they likely still work with their soon-to-be-former boss, who likely still works at the same company the candidate is leaving.

This is the most common case, by far.

Are you suggesting going further back, to the candidate's previous previous employer, and hoping that their direct manager from years ago is able to provide reliable and accurate testimony on the current state of the candidate ?

I said ex-boss, not current boss.

I'm surprised this is not a universal experience based on these responses. Every candidate I've interviewed has given at least one ex-boss as a reference. Every reference I've ever given has been ex-bosses.

Do you believe someone who hasn't worked with the candidate in years is a useful reference for the candidate today ?

Yes, of course. Their personality hasn't radically changed from a couple years back.

What you see in the CV is the name of the company they worked for, not the name of their direct manager.