A recent post about slash chords got me started thinking about whether it is technically "correct" to use a slash chord to mark a root position chord. For example, in C/E, the root is C. In Gm9/F, the root is G. In general, slash chords follow the pattern A/B where A would be the root of the chord and B the bass note. In other words, slash chords usually denote inversions. If I had to guess, that meaning probably comes from the old figured bass notation.
However, slash chords are also sometimes used to indicate not only inversions, but also marking specific voicings with upper structures. For example, C/D. The root of this chord is often not C, but instead D, in a D9sus context. In this case, the slash chord notation isn't so much of an inversion as it is an indicator of a voicing or a selected interpretation of the chord. The same logic can also be applied to other slash chords where chord A does not normally contain the note B: Em/C is a way to express Cmaj7, even though Em does not contain a C.
My current understanding on slash chords would be best stated as:
Slash chord notation is meant to signal to the reader that the bass is acting independently of the rest of the chord above.
I have seen a few sources online define slash chords more strictly such that the root note cannot be in the bass:
"A slash chord [...] is a chord whose bass note or inversion is indicated by the addition of a slash and the letter of the bass note after the root note letter." - Wikipedia
"In simple terms, a Slash Chord is a chord where the bass note – the lowest note heard in a chord – is different from the root note." - Hello Music Theory
"A slash chord is a chord which indicates emphasis of a bass note other than the root of the chord." - StudyBass
Emphasis above is mine. I recognize that these are not among humanity's most reputable or rigourous sources, and that not all sources agree with that definition. However, there's something to be said for a number of top search results all turning up this non-root-bass definition, and I also did interact with at least one person on this site echoing similar sentiments. In fact, the Wikipedia article makes the claim that slash chords can be the bass note in easy arrangements "to avoid writing chords more complex than triads" for beginners, implying that this is not the intended usage of the notation.
I already know that both usages of slash chords are common. My question: is using a slash chord in this voicing/independent bass context an accepted standard concept for chord symbols? Or is it more of a fast and loose shorthand notation that is (getting) adopted for convenience? Should Em/C always be taken to mean E is the root with C in the bass, or should it be equally valid to use the symbol to write C as the root and Em as upper structure?
Bonus points: for those in the "accepted and valid notation" camp, is there any difference between the chord symbols Em/C and Cmaj7 (or C/D vs. D9sus)? I would assume the stricter "inversions only" theorists would argue that there is no difference whatsoever. Would the "inversions-only" camp accept any bass notes that are not in the upper chord (e.g. G/F vs. G7/F)? My thinking is that the looser group is fine with either notation.
I did read this very similar question. However, from what I can tell, that question asked whether it could be used. My question hopefully is more about whether it is technically correct notation vs just a common shortcut.