My advice isn't Pollyanna advice. I have worked at Fortune 10 and Fortune 500 firms as well as in startups, plus I have run my own company for years now. In all those places I have held senior positions at one time or another. So my experience is real and practical. In the larger firms generally managers and above are trained to redirect all reference requests and employment verifications to HR, especially for employees who were terminated. This is a risk mitigation procedure by the companies, as lawsuits are real. And generally past employees do find out when employers try to damage them because of friends or sympathizers still at the company will reach out and let them know.
You say back channels are the way and the good ole boy network still exists. I agree to a point, but practical experience again is showing me that even using people I know they are reluctant to answer questions because they fear reprisal not just from lawsuits, but for what should happen when they need a job (direct quote from someone not too long ago). Today's person you fire could be your boss shortly down the road, so I think this contributes to why generally I find experienced people reluctant to give any real details. Exceptions happen though, and in cases of theft, arrest etc I have seen more direct responses since there is generally public record, but again, that is a very low percentage of times. Also, as @HillRat said, SMB's can be more volatile since they generally lack professional HR and so that is a little more subject to the good ole boy network and off the cuff comments. Or the pregnant pause as he also mentioned.
Personally in my current and past businesses, we will validate employment dates, salary if they already have the number (otherwise that is the employees personal data), job title and what the job position entailed. We generally give no indication as to why the person left or whether it was voluntary or not. In large organizations these types of calls go to an HR group or even to an outside firm that only has 4-5 pieces of data on people. Home Depot Supply at one time had an automated system that handled all verification of employment calls to prevent mishaps from happening. I have no clue if this is still the case, but it was creative.
If an employee leaves on their own, or asks me personally for a reference I will make a judgement call about if I will do it, how much to say and how to handle it. But in the cases where someone was terminated they rarely list that manager as a reference so those calls come from a verification of employment perspective, where we answer the bare minimum.