Because Allura was cold and distrustful towards Keith after finding out he was half-Galra, her detractors constantly portray her as a hateful, racist shrew who would physically beat Keith or verbally abuse him every chance she got. Never mind that her bitterness didn't last long and she did realize on her own that Keith was still on her side, before apologizing to him.
Pidge and Colleen are shown having a hilarious exaggerated fight in a flashback during "Launch Date," in which Colleen has grounded Pidge for disappearing for so long and worrying her. It's clearly exaggerated for laughs and has no real bearing on their loving relationship, but fans who project onto Pidge took it to mean Pidge resents her mother and that the two have always fought even before Kerberos.
Lance is bashed heavily for his Casanova Wannabe behavior towards Allura in the first two seasons, even though seasons 3 and onward showed him outgrowing such behavior and learning to appreciate and respect Allura as a person. Naturally, it was mainly Lotor/Allura shippers who treated him like this.
Even after undergoing some major character development, Keith detractors still see him as the emo jerkass who was such a shitty leader he almost got the team killed at least twice and then ran away from them to join the Blade. Some Shiro fans also never forgave Keith for "stealing" the Black Lion from him-even though it was Shiro's own wish that Keith take over if something happened to him.
On the flip side, the rest of the Paladins are still accused of "chasing Keith away" and being "abusive" to him after the episode "Code of Honor", where they expressed their frustrations with him spending more time on Blade of Marmora missions and not with the team. These fans seem to forget that Keith was voluntarily pulling away from everyone else and they didn't know about his issues until he came out and told them, and that the others immediately softened up and stopped being mad at him once he did explain things.
Worse? After the end of the series, "Keith stans" popped out of the woodwork to use this episode to bash Pidge, who went off to find Matt in the very next one. Never mind that some time may have passed between episodes, that she told the team where she was going, and that she'd planned to find Matt after the battle with Zarkon. Or that Matt ended up being a good addition to the team while he was there. Instead, these overprotective Keith fans accused the show of singling out Keith for punishment while giving everyone else a free pass. (Or rather, Pidge and Allura a free pass because they were girls.) Some even tried to cite "Fall of the Castle of Lions" as further hypocrisy because Shiro let Pidge leave while scolding Keith for being mad about it...when Shiro's behavior in S4E1 was actually a major hint that Shiro was Not Himself at the time. And that Pidge came to the conclusion on her own that she did want to help the team, and Shiro likely knew she needed that time to herself to figure it out.
The show itself will always be remembered for having one of the most toxic fandoms in existence, including shippers who blackmailed the studio to try to get their OTP made canon and threatening an actor's wife and children for said actor voicing his support of pairings they did not approve of.
The creators will never live down being "liars" and "queerbaiters" for not giving the fandom exactly what they wanted... particularly, their OTPs. Or for "fridging" Allura, despite her sacrifice carrying zero earmarks of the Stuffed Into the Fridge trope and the creators explicitly saying she wasn't gone forever.
Sheith fans are especially bad about this. Every time an LGBT cartoon couple becomes canon, they barge in on the fandom yelling and screaming about how their ship should have been canon. Discussions of LGBT cartoons can't be had on social media without the shippers whining about Shiro not kissing Keith. Especially in the She-Ra and the Princesses of Power fandom, where they actually guilt-tripped Catra/Adora shippers for being happy they got their ship because Sheith wasn't canon! Even with the Fandom Rivalry between Voltron and She-Ra, they were two different shows. One pairing had nothing to do with the other.
A lighter, more affectionate example is Shiro's Gary Stu in the Monsters & Mana episode. Many Shiro fans loved seeing him act like such a self-indulgent dork, right down to his Stu having a Backup Twin with a near-identical name.
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