edit: Just noticed this is probably slightly off-topic, as an answer, but given the question was asked... Further discussion about this should be elsewhere, I suppose?
The game currently is voluntary donation where you tend to get personalised thank you gifts from the creator(s) if you do. The Steam (and Itch.io) version is basically a different opportunity to voluntarily donate with in game bonus material shared across all those who subscribe that way.
(That's the 'customer facing' description of the system. It's also got developer[1]-facing costs/benefits that are just as different to the up-'til-now sole distribution model, and then there's the middle-men that never existed before who add value by 'footfall' potential, doubtless repaid bynsome form or other of commission/cross-fertilisation of their respective portfolios. Steam seems to not have an "everything must profit us" mentality[2], but work more as a Venture Capitalist concentrating on advertising/distribution aspects of their 'investments'. I presume they like it if on balance they are part of more fly-off-the-shelves successes than outright flops and failures.)
Maybe think of it as a "Freemium" software, but without any of the usually implicit Pay-To-Win/Ads-To-Advance aspects?
[1] You can newly-include the artists in this category.
[2] I've never really liked my early encounters with Steam, in different contexts (perhaps before they changed to encompass a significant amount of this sort of more altruistic-looking scenario), but it certainly intrigues me enough to consider this as my future donation route, cutting through my age-old cynicism. We'll see.