That's fair. I do it in-game because I like it, though ofc. not to the point of dying or taking free harass for it. Regardless, it's not mandatory.
What's sportsmanlike in League? Lots of stuff! Being a graceful winner and loser, complimenting your opponents on plays well-done, and just... idk, interacting with them in a nice way relative to the game? It's hard for me to define concretely off the top of my head, but it's the same as sportsmanship anywhere else. Play to win, and talk to make them less salty about it. When you do lose, accept it gracefully. Compliment people - friend and foe - on things they did well! Accentuate the positive, esp. to people who might be tilted anyways.
Mind, I do a lot of it naturally, since I have a somewhat sunny personality (well, most of the time), so I often underestimate the struggle for people who are more competitive. I realize that it's not just something as easy for everyone. That's not a snarky thing or bragging or anything - people are just naturally different. I'm happy, I have strong game knowledge, but I'm honestly pretty terrible at actually playing League. People with better instincts and faster reaction times completely wipe the floor with me. Each person has their own strengths and weaknesses. A lot of success is learning to play to your strengths, shore up your weaknesses, and avoid things you're not so good at... unless they're fun and you don't mind being not so good at them. :P
But yeah, just be nice, go out of your way to say positive things, and rephrase and reframe negative in a constructive light. That doesn't mean negativity is inherently bad - sometimes it's good - but always treat it with great care. Doing so does wonders for your own team morale (which is a huge and badly underestimated resource) and for everyone's general satisfaction with the game.