Almost entirely signal boosted as proof of a carrot that both will most likely never arrive, and most likely won't change anything even if it did arrive. There's a large difference between individuals pushing for something and them actually managing to get them passed at a scale that matters.
the overton window aka “the window of discourse” is literally about talking, and making things more acceptable by their existence in the public consciousness
the more people TALK about alternative voting mechanisms, the more likely it is for some change to occur… 1 person can’t just up and change the system overnight, but by “signalling” as you’ve dismissively called it, it makes it more likely that other people talk about it and that makes it more likely that it becomes policy
How does "signaling" translate to policy? The majority of Americans want abortion codified, legal weed, single payer healthcare, increased gun protection, and more. Why do you think these haven't passed?
Depends on what you mean by "leftist values." The closest is probably Rashida Tlaib, but in general "The Squad" consists of Social Democrats, which I would consider Center-Right.