Does the UK no longer stamp passports with a small stamp?
Background. When an entry clearance application was received, a blank page would get a standard 'received' stamp on it. The stamp contained the date and the post number. If the application was approved, the entry clearance vignette was affixed over the 'received' stamp and end of story. For refusals, an assistant used a straight-edge along with a black ball point pen and drew a line through the 'received' stamp from one side of the page to the other. Nothing else was done to the passport, and there were never any comments or other indicators.
Looking for an answer drawing from credible and/or official sources.
There are no 'official' sources to rely upon for this question. If it existed, the information would be found in staff training materials which are not available to the public. It means the answer to your question relies solely on credible sources. So my credible sources are:
Both have represented casework in the last month where the person had a recent entry clearance refusal. I checked with both of them today.
The consensus is that the practice of using refusal stamps has stopped, but intermittently so. Some posts are still doing it, apparently at the direction of the post Entry Clearance Manager (presumably for situations where biometrics were not captured or for non-biometric passports).