Forgive me if this has been said repeatedly, but just in case it hasn't:
Steam was never mentioned during the Kickstarter or Slacker Backer campaigns. I'm not going off memory; I've checked, and anyone can easily do the same (but countless out there haven't and won't).
The first reference to Steam came in a June 2018 update announcing the backer surveys, listing the Steam client among PC requirements that were explicitly subject to change during development.
The labelling of version options in the survey itself did overtly specify Steam, though by this point most if not all users would have already donated without anything resembling a promise of this sort.
I say most if not all, because I have no way of checking now what the wording was in the upgrades/add-ons process, which may or may not have labelled the PC rewards as being Steam-based before the point of donation.
If it did, then people who added or upgraded to a PC reward during the backer survey period may have a claim to that promise being broken. But they will be a slim minority at most, and unlikely to include everyone currently claiming they were "scammed."
Also lost in the uproar is the fact that Steam pre-orders never opened. Yes, for months now it's been formally advertised that the game was definitely coming to Steam on release, and that is justified cause for disappointment, even frustration. But again, not for refunds or lawsuits.
Kickstarter is knocking back any reports it receives because no campaign promises have been broken. People can be upset that it's not coming to their preferred launcher, and if they added/upgraded during the backer survey they might have cause to demand a refund, but the vast majority have no grounds on which to claim compensation. They're not getting what they want, and that's not good, but for most or even all, the terms of their financial contribution did not promise or even hint that it would be on Steam.