Agreed on chord numbers and progression being the analysis that should have been done. For example, blues is mostly defined by a 1-4-5 progression and the ol 2-5-1 is pretty ubiquitous across time and genre.

Also, I think disappearance of 7th chords - major, minor, or dominant - is vastly overstated. Keep in mind that these are from guitar tabs so likely ignoring chord inversion / voicing / substitution taking placw to simplify notation. For example a B minor triad can be substituted for a Gmaj7.

Bm triad = B,D,F#

Gmaj7 = G,B,D,F#

Or if you want to be fancy a Bb/Gm can work as either Bbmaj7 or C7 depending on where you put it in a progression.

As you have suggested, it has also become common to use patterns like Bm/G to create a Gm7 that is less spicy than if the bass G were mixed into the treble octaves. 9 and 11 chords are also done this way.

C7/D is a C9 chord, and C/D is a bit more "open" of a sound but still a 9 chord.

G7sus4/B is a G11 chord, dropping the 9th.

Anyway a 2-5-1 is the rotation of a diatonic substitution of a 1-4-5 (2 for 4). Only one note difference between those two chord changes.

>blues is mostly defined by a 1-4-5 progression and the ol 2-5-1 is pretty ubiquitous across time and genre.

I IV V, and ii V I, to be clear.