Basically it’s allowed but they must properly mark it as NSFW, so it can be filtered out for minors and people who are actively trying to avoid it. More or less they’re making the sharing of adult material a more legitimate thing for twitter, which is kinda hard to tell because twitter has always been filled with porn but I guess it’s just more official.
Wherever or not people will actually mark their content as NSFW or if Twitter will enforce it is in the air. I’ve seen Twitter content sensitivity setting used more for video game spoilers than it’s actual intended use.
I’d argue that Valve does more than just take 30% as a middle man. Between Steam Input, Proton, the beta built in recording system, the Forums for every game, community system and the marketplace, having your game on Steam is a massive value generator for the consumer and by extension developers.
30% might not be what the industry standard should be, but Valve isn’t just providing a standard digital distribution service.