I discovered a command
System`Private`ValueList in my
.mx dumps. It's an internal command and thus undocumented, but from the little I could find online I understood that it used to be available in
System` prior to version 3.0, albeit still undocumented, and used sometimes by package developers for input and output formatting (1). In 2 it is said that it is now accessible through the Private context, which is indeed what 1 is doing in a simple version check.
Interestingly, 2 says that "a certain amount of misinformation has been posted about it" but I can't seem to find what they are referring to, or I can't identify it as a misinformation. The most comprehensive review I found is 3, but the examples no longer work in 10.2 even after the change in the context.
Edit: They work but slightly differently, thanks, Mr.Wizard! This does not affect the main question that follows.
I wouldn't need to care but 3 says it can do something "you cannot achieve using the ordinary built-in commands", and this intrigues me. Does anyone know what all the talk about the similarity and difference from
HoldForm is about, and what's the command's purpose? Internally it seems to be used rather often.