If you can give me examples where you think things clearly changed drastically then I'll be convinced of your position. People got faster and their understanding improved some, but not to any extreme degree. People are better at SDI and shield dropping now. But stuff like Sfat learning to camp Hbox like people who beat him already do isn't new it's just his way of doing it for example. Now player matchups could change a lot like what happened with M2K Armada but I don't think there was a lot of new ideas there just some small refinements.
Examples that come to my mind are:
Fox v Jigglypuff:
I think since Hungrybox's "ascension" a lot of development has happened here, maybe the matchup is the one which developed the most in the last 2 years (it is still relatively unexplored compared to many others, but it was way worse before that). We see multiple different approaches from different Foxes, and several of them do very well (after all, it's a good matchup for Fox). The concepts that looked most convincing to me abused Fox's clearly superior vertical movement, not trying to beat the bair horizontally any more because it is a mixup that is often disadvantageous. Also there are a lot of different forms of positional advantages, and seemingly neutral looking gamestates can be favoured for one side, and players have become better at recognizing this. To give an example, Armada often allowed Jigglypuff to short hop or full jump towards him from across the stage in the past, which leads to Jigglypuff being able to force Fox into the corner with the bair threat zone, and even this is not always safe if Jigglypuff goes for the read with pound. In more recent sets he will use fj nair to challenge the jump by using it preemptively, and if you fall down on Jiggs with it you beat bair. Alternatively, he will only retreat as much as needed in this specific moment and full jumps out after the landing, so bair can't hit anymore and grab not yet.
Plup:
Plup innovated a lot recently. He created a few blueprint sets for Sheik vs Jigglypuff which has never been played at a this level before (although Shroomed had some close sets against Hungrybox). In every matchup, his skirmishing is very refined, to the point where I think it should be considered a big leap, but I haven't looked into it enough to be able to tell what exactly makes it so good.
ASDI down/Crouch-cancelling:
Just in general, but especially Falcon players, because the character is good at it, and he needs it. Not sure if all of this is new, but at least it is applied much more. I've heard of players holding c-stick down during dash dancing when at low %s to option select it every time, which might be new. I think there is also a lot of potential for CC even when the resulting frame advantage isn't enough for a punish, because they will either be at risk of getting hit or will have to commit to something that could get whiffpunished, but I haven't really seen that enough to count it.
Defensive maneouvres:
next to SDI which you mentioned, slide-off DI has improved a lot (against upward attacks at low %s, in platform techchases, against low angle attacks at higher %s), as has teching when recovering.
Gameplans:
In terms of gameplans, in a few matchups (non-dittos) where it was common to see both players dashdancing and mixing in some moves of their neutral game arsenal here and there, this kind of neutral game gameplay can be advantageous for one character, often the faster one at higher range and the one with the more fearsome neutral game tools (like Marth, Sheik with grab, grab/dash attack) at mid-range. To avoid being in mid-range too long, fast characters have shifted away from DD+SHFFL gameplans in many cases, instead trying to close the distance asap. Fox players now often just dash in completely to avoid standing in these threat zones for too long, while using some other option (like a dash in->crossup full jump) to be able to punish a defensive hitbox. I'm not sure how much of an innovation this is, because maneouvres like running shine definitely exist for ages, but from my point of view the understanding of the neutral game has changed in some way. The same thing can be seen with Captain Falcon, although it is usually combined with aerials and often the hitbox s just "sprayed" to cover one or multiple dash patterns, preferably dash away, which often acted as a get-out-of-jail-free card in the past.
I have to admit though that Falco and Marth development definitely suffered from your absence
Please to correct me on anything if anything is wrong, I'm not 100% sure about a lot of this as they are mostly own observations that could fall prey to all kinds of errors and biases...