M.E.T.A., Trolling, and the difference.
This is to inform you of Riot's stance on the topic. This is not my opinion. This is the fact of what counts as trolling.
#OFF-META IS NOT REPORTABLE, BUT TROLLING IS NOT OFF-META
For those not aware, the term meta refers only to what is currently (read "during this patch") the strongest option. It refers to both champions and items, and can very seldom refer to roles.
#Meta
For example, currently according to op.gg Kog'maw is the best ADC at Diamond level, with a win rate of 53%. And his meta build is on-hit Runaan's + Guinsoo. Kog'maw currently meta, and he most effective with that build.
Conversely, the worst ADC this patch is Jhin. This means Jhin is currently off-meta, but his most effective build is Youmuu's>Duskblade>IE. Which means that your best bet is to build that way, but even when you build that way your chances are lower than Kog'maw's because that build is only going to put you on par to achieve his 46% winrate odds against a team of equal skill.
#Trolling Now, what does this have to do with trolling? Well, some people who troll pick like to defend it as "off-meta", which is simply ignorant of the term. As I explained with Jhin, off-meta only refers to a champion that is not currently the best option, with their highest-performing build and role performing worse than the others choices.
It does not refer to champions being picked for roles and/or with builds that their stats and scaling are not designed for.
For example, Gangplank is currently the best performing top laner with his standard Trinity build. However, just because Gangplank is the best when built correctly does not mean he retains that performance when built full AP (or 6 boots) in the bottom lane. Doing so ventures out of the realm of off-meta and into trolling. Of course, the order of your items and your finished build will change depending on how the match is going, but building an AD-only champion like, say, Riven with full AP items is inexcusable for anyone past level 20.
In conclusion, off-meta does not mean the same thing as a troll pick. Playing to win, if it isn't as blatantly obvious as I know it is, includes champion select and your build. It's perfectly fine to pick an off-meta champion if you feel you're better with or simply want to practice to get better with that champion because their style suites you. What's not okay, and I mean objectively not okay based on the rules presented in the Summoner's Code, is to troll by doing something like picking Jhin for ADC but building full AP.
If you're curious as to how that's trolling, I'll refer you to two other cases that set the precedent for why this goes against the Code. In both cases, and in any similar case of troll-picking/building, the issue is not the build itself. It's in the lack of communication that you intend to play a different role than what the champion is best at. In these examples, both involving the "Support" not communicating to their team that they were instead going to bring smite and take the enemy jungle rather than stay in lane and actually be a support-- which is akin to stealing the role of the jungler. And now with the new champion select, such things simply look like you're angry you were autofilled. https://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/79aGu3wR-support-singed-isnt-banable-but-thats-not-the-point https://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/FrILlWp7-14-day-ban-because-playing-nunu-support-with-smite-is-stealing-other-peoples-roles For my examples, AP Gangplank or Riven isn't just role-stealing. It's closer to leaving, since there aren't roles that Riven or Gangplank fulfill when they're built full AP.
Edit: I'd like to point out one particular that a comment mentioned, and refute it.
He claimed that meta is popularity based, rather than effectiveness based. I argue that that's flipping around the cause and effect. A champion does not become meta after being popular. A champion will however become more popular if it's found that new changes have made them meta.
For an example of a champion who's almost always both popular and meta we have Lee Sin. He has been consistently strong and fun to play ever since his release, especially considering his mobility given the state of mobility in League at the time. And as we'd expect with the meta/popularity model that I propose, his popularity declined when he wasn't meta--- most notably during the lifespan of Sated Devourer.
Then for a current example of a champion becoming meta and gaining popularity, we have this patch's Nunu, who jumped up from last place in jungle pick rate to 13th because his new changes made him the number one in effectiveness. So good, in fact, that his "counters" are still losing to him more often than winning, while champions he counters are losing to him 64% of the time.
And finally, as an example of a champion who is meta but not popular, we have this patch's Aurelion Sol. He may be the best performing, but his kit is still unfun, leaving him less popular despite his strength.