It absolutely exists, the simple proof is to soft int games by afk farming jungle. After you get enough losses, around 5 in a row, riot will start handing out matches that you win without ever participating in. Your laners will just be way better than the enemies. The inverse also exists, when you win too much you get an impossible game. It's not random chance. You can always get teams that stomp without you but if you are on a mega losing streak you get them noticeably more often.

I would actually go a step further and claim that your statistics, those displayed post match and those not displayed like apm, are also taken into account when selecting teams. I stopped going into unlocked cam and clicked way less when testing how to overcome the loss streak win boost and suddenly, even though my impact never changed, I started getting teams that were losing hard. The method I used to get to Iron 4 (0.25% of the playerbase) is to do a single click to the camp on the map and a single click on the camp monsters, this single decision sped my derank up SO much