He actually remained relatively untouched throughout the updates. Most of his changes are down b and up changes (the distance they go, not power). In 2.6, his up b had less landing lag than it does now, and down b had less end lag, and 3.0 retained both moves. His mobility both ground and air were the same as they are now (dash dance, wavedash distance, wavescuttle, platform movement, etc). The only real difference I can think of off the top of my head was his dtilt, which popped them up like in melee. In 3.0, his up b didn't go as high, and his dtilt became a 15% mr saturn (except not as much shield damage, but still a considerable amount). Side b sweetspot was always the same throughout every version thus far (at least 2.5 and up; I have no experience with anything before).
In 3.5 the only buffs he got were indirect, and that's mainly due to other character getting nerfed and the dpad down b, which was the result of adding simultaneous button press for 3.5. The only direct buffs he got were dash attack (which is now safe on hit and does a ton of shield damage, plus actually knocks the opponent back a little) and uair (which has two more active frames). Down b got nerfed because it has more endlag and thus cannot really follow up with anything threatening, thus making it a less safe approach option (3.0 down b was mad stupid because it had almost no end lag and could follow up with up b and down air, nair, etc). The only reason as to why the down b nerf is negligible is because of dpad, which in turn makes it even better. I think removal of the dpad trick will make his recovery pretty easy to gimp. The only other changes he got were the green effect on fireball, misfire, and up b sweetspot, which was simply cosmetic. And fixing a few glitchy things like the hitlag on down air.
The reasons why Luigi is doing well now is because more players are using him, and thus are finding newer things with the character. MU inexperience also seems to play a factor as not many Luigis made it to top 8 or grands, etc, in previous installments. Aside from Eli and Vist, there really weren't many good Luigi's either. And this is noticeable in the match with Dong and oracle where Oracle's di button just stopped working when he was getting combo'd into aerial up b. He also got hit by jab up b's (which should never work unless the opponent is a little above ground). Sethlon did not know that he could crouch cancel down b and punish the end lag either, among other things.
@
Foo
What do you mean by "dejanked"? Is Luigi's long wavedash "janky"? Or perhaps is it the down and side b? (I can probably agree with the latter two, but I don't know what you really mean by "jank")
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Ripple
it does not. There is no intangibility for his head on upsmash in this game. It was in melee, however, so I can see why you'd be confused. It has a bit of range but that's really it. You can check debug mode to confirm.