TL;DR at the bottom
In the past week I've met one player who was most certainly lagswitching (causing the game to lag badly during an online Quickplay match to the point where the framerate is so low that both players have infinite time to react to the current situation) as a Mewtwo vs my Wolf at around 8.5mil GSP:
I was fighting him quite ruthlessly as Wolf, plenty of blaster use to establish a percent and stock lead so that I can spam sideB at my leisure, after about 60% on myself and 95% on the Mewtwo, suddenly the game begins to lag and has long pauses between actions happening. I felt an immediate urge to just waste all my stocks and get out of the game as soon as possible but I tempered that idea and chose to stay in the match in the hopes that the lag would clear up.
I took this Mewtwo's first stock with upsmash out of shield, but this was when things became far too obviously contrived in regards to the lag:
Either the Mewtwo player themselves or another person in the room with them were trying to sustain the lag during moments of my advantage. It was comical, the lag would stop and start the moment I was in control of the stage, but they weren't very good at timing it right.
I lost neutral a couple of times and was on my 2nd stock by this point. As I was convinced of the lagswitching I chose to play the most frustrating game I possibly could for my opponent(s), turtling and making Mewtwo chase me back and forth over the Jungle Japes stage and punishing his attempts to evade the blaster or chase me down. Naturally I was crouching a lot between lasers.
So as the Mewtwo is recovering, he's used his 2nd jump and is ready to teleport, lag starts up again and teleports backwards away from stage and loses his 2nd stock. He disconnects fully right after he teleports away, a victory to be savoured.
Second
Today during Elite Smash on my ZSS, fighting a Ness player. He's got some mad control of Ness' double jump movement and aerials but also seems to be lackluster defensively and quite easy to hit. I can't claim I would've won the match without any lag, nor am I 100% certain (in hindsight) that this was a lagswitch deliberately, but I was up a stock when a more mild but very consistent input-eating series of lag spikes would occur until my opponent took the lead, and then they stopped.
It was the kind of lag where I would attempt to dash attack quickly but get an ftilt as the lag had eaten my dash input and I was still holding forwards.
I took the Ness' first stock quite convincingly and the lag started. The Ness takes me to my last stock and the lag vanishes again. I start making a comeback, taking his 2nd stock but still at a fair damage deficit and the moment he respawns, the lag returns.
Eventually we end up last stocks each, 100%+ each, he caught me with an aerial and took the match.
I wanted to match against someone else so declined to rematch but that just brought me into another game with the same Ness.
It was the Hanenbow stage and I was picking up my controller a little late, so I just avoided the Ness player and kept hitting the leaves with him; as if he had accepted my percived lack of morale to mean I wasn't going to try to beat him. I started taking potshots at him, he started to chase me, I started to punish his chase attempts and took a stock, he took a stock, I took another, 2-1 to me. I think he didn't notice the timer until it was at 45sec remaining. There was no lag this game.
As I was sure he was lagswitching, again I relentlessly crouched at him as the 5 second timer was announced. He didn't want to rematch me.
Regarding this Ness player I encountered (9.6mil GSP), there are a few reasons I'm not sure he was lagswitching (may have been a legitimate player)
1- He didn't rush me down and take any stocks while I was away at the start of the match
2- He moved extremely well during his offense
3- If he was a legitimate player and I'd just cheesed a timeout game from him with some hefty squatting thrown on top, I wouldn't rematch me either
Reasons I think he was lagswitching
1- The lag worked very consistently in his favour during the first match we played
2- He didn't seem defensively conscious enough for a 9.6mil GSP Ness player? I'm speculating at this point
3- The second match, he may have thought he didn't need his lagswitch to beat me until it was too late
If that Ness wasn't lagswitching I may have made a poor judgement call...
TL;DR - Fought a definite lagswitcher who's lagswitching backfired and caused him to ragequit, fought a suspected lagswitcher and lost, then timed him out in the rematch
Finally if you do meet with a suspiciously laggy situation online, don't throw the match! Instead accept what may be a loss and fight it out. Players who artifically inflate their GSP beyond their actual skill level can be caught by some of the oldest tricks in the book. For the first kind of (definite) lagswitch I encountered, having so much time to react and make the right choices with my gameplay enabled me to still outplay the cheater, which was hugely satisfying.
If you do encounter a match that's too laggy to continue, or if you've just had enough of a certain player and their rulesets matching with you and wasting your valuable gameplay time, you can block them from your profile screen on the home menu.
The reason I'm making such a big deal about the second type of lag I encountered, it was different. It was still stopping and starting relative to my opponent needing the help, but it wasn't severe enough to slow the game down to the point where the framerate was non-existant. It was like being hit by the beehive item, just annoying little jabs to drop my inputs.