A lot sword hitboxes, including Marth's, had a vertical shift (assuming the sword is in a horizontal position), I believe mostly downwards.
This changed the way he had to space tippers, I'm not sure if it's universal, but anecdotally I'm pretty sure this is the case for fair/bair and it has been shown this way on nair. Up Tilt and the like are weird but I'm not sure if it's the exact same.
I realised this a while ago and have generally adjusted my spacing to be mostly consistent. However, there are some hiccups. The problem is also exacerbated by the inconsistency of c-stick for extra directional inputs you do not want.
The combination of the two have great consequence on Back Air, altering how the move worked at it's peaks (first and last few frames), that are where the move was predominately aimed to hit.
In Smash 4, as a result of the animation and centred hitbox placement, the move was difficult NOT to tipper on the rise verse a grounded opponent (near point blank oos bairs was a broken option for Marth). The tipper hitbox protruded a lot more horizontally at it's lowest vertical point thus its ease for tippering on frame 7.
With the lowered hitbox, I would imagine the extent of area of which the tipper hitbox was horizontally on f7 is smaller, and in practice this seems to be the case - Marth's rising bair can whiff shorter characters or sour spot while in smash4 I'm sure it would be 'easier'.
In contrast, the other 'optimal' spacing was the final two frames (perhaps they reduced bairs hitbox length by 1 frame too?), that in a similar fashion to the first frame essentially auto-tippered people above you as you rose or they fell towards you.
tl;dr Marth spacing tipper aerials was essentially solved late in Brawl's life time and through standardizing hitbox placement (in Brawl basically every move was different) it became even easier in smash4. For Ultimate they recognised this and did the shifts (I await full hitbox placements for a lot of my characters tbh), plus added semi-RNG to tilt stick. Joy. For forward air it's fine (IIRC this is how it was in Brawl) because lowered hitboxes work for its overhead animation, but for upper cuts it's generally counter productive.
I could foresee them increasing the rewards of tippers over time.
Or maybe they can just shift back air hitboxes back, this baby not being our easy bake auto tipper killing machine really sucks.