So I spoke with Perhapsman about Link's d-smash. This picture will be helpful for explaining things:
There are FOUR distinct hitboxes on Link's first hit of d-smash.
Going outwards, the hitboxes deal these damages:
16%, 17%, 16%, 13%
The hitbox that is used when multiple overlap with an enemy is always the one more outward. This means that being very close to them so that the back part of the 17% hitbox (this is Link's forearm and the sword hilt) and the one inside Link are both overlapping your opponent gives you the best shot at landing the 17% sweetspot (since the 17% hitbox will take priority over the 16% hitbox).
Against Fox, the 17% hitbox gets a knockdown at 11%. That is WAAAY lower than I thought haha. It is 12% against Falco. This is the hitbox you want for tech chasing them and it will start working very early. Against Falcon, the 17% hitbox doesn't get him when he is standing (due to some derpy Z-axis BS lol) but the 16% hitbox will get a knockdown at 18%. This is assuming no true CC (as in crouching before getting hit).
The 13% hitbox gets a knockdown against Fox at 22%. Still better than I thought, actually, but it is quite a bit higher than the 17% hitbox. This is, again, assuming no true CC.
Seems like Falcon is too wide when standing, so the outer 16% hitbox always hits him (the middle part of the sword hitbox).
Numbers for true CC:
Fox (17% hitbox can't hit him for the same reasons as falcon, so 16% hitbox):
42% - This is more how I thought it was haha. So true CC is still really strong against the d-smash.
Falcon (still the 16% hitbox):
54% - Daaamn.
Anyway, what's important is the difference between true CC and not. If you are tech chasing, d-smashing the tech in spot will get another knockdown very easily.