There are a variety of wacky arrangements that could theoretically be constructed. The least wacky of the wacky would probably, genuinely, be to do something with a stablecoin (ie the ones with a value pegged to the dollar), as crypto, and those specifically have just become MUCH more clear and government approved in a regulatory sense. It's becoming increasingly easy to just buy crypto with a credit card, on ordinary sites and via ordinary companies rather than having to go to some weird part of the internet.
But any wacky system means a lot more work for customers, work most of them aren't interested in. And from a platform perspective, Itch is too poor to be able to, and Steam is too rich to care. They could nuke all their NSFW stuff tomorrow and it wouldn't hurt their bottom line in any significant way.