doesn't it hold that when the multiple scalars produced by slicing an array is pushed onto the stack, it also becomes a list? And that all but the last element of that stack-held list of scalars are then discarded when in a scalar context?

So whether an op returns a list or not depends on whether it creates a list internally? It doesn't makes sense to describe the result of a function using different terminology based on whether it builds a list internally or not in order to return that result.

For example, say grep built a list on the stack even in scalar context in an old version of Perl. According to what you said, that grep returns a list in scalar context. That means grep returns a list in scalar context in one version of Perl but not in others, despite returning the number of unfiltered elements in both.

Is it such a "bad meme" to say: A slice results in a list. And a list in a scalar context will return the last element of the slice?

Yes, cause it commonly fails. In the aforementioned version of grep, grep results in a list. And a list in scalar context will return... the last element of the grep? No, it didn't.