If you observe a game it's extremely easy to tell if somebody is feeding or not, using the metrics the game records it becomes more difficult.
I think automatically picking out the most egregious cases of intentional feeding would be extremely easy, but there are some games where it could be tough to do automatically.
Take
this match for example. Gnar intentionally fed after a couple of deaths, and continued doing so until it became clear he couldn't stop us from winning (the final push). He would run directly up a lane, sometimes ignoring minions, and see if he could win 2v1s or just 1v1 people he obviously could not 1v1 (Vayne multiple times, including immediately after dying to Vayne once).
His score is bad, and his deaths are unusually high for a win, but he never says he's feeding in chat (he rages the entire game, complaining about our jungler for not saving him from getting camped and saying we all suck, the usual) and his score isn't that shit.
However, Gnar suddenly dies 5 times in ~5 minutes (around 17:00 to just after 22:00). That is something that a terrible player will not do past 15 minutes (death timers alone make it so unless you're running full speed at the enemy base you won't die that quickly).
From the score Gnar could just be a bad player, but seeing how quickly he starts dying it's clear that he tried to throw the game.
There are metrics that can show somebody is feeding aside from just score, and it can be pretty fucking clear. Nobody dies that quickly unless they're intentionally feeding or are pre-30. I'd hope any automated system would be more lenient with lower ranks and stricter with higher ranks, because there are definitely people who WILL return to a lane that's getting spanked and be genuinely surprised when they get killed over and over again.
Re: Why is feeding bad?
Feeding not only gives the enemy a pile of gold, but it also means your team is essentially down a player. Everyone gets pissy and the chat becomes a wasteland of people yelling at the feeder while the feeder either yells at them or stays completely silent in the hopes that they won't get banned.
It's usually a waste of 25 minutes (let's count champ select and load times!) to have a feeder in the game, no matter what team you're on. You do gain LP if it was ranked and the feeder was on the other team, but it's still not a fun game. It'd be like in American football if the other QB intentionally threw the ball to the defense every single play.