That would suck. Steam doesn't really allow anything good as far as AVNs. They're even worse than Patreon. Good way to lose Patrons that way.
If Patreon could ever match Steam then it could be a problem. But when it comes to finance, well, let's just say Patreon brings in some small change for devs compared to Steam.
Check Oppai-Man! They have 7500 paid members on Patreon and earn around $35K there, meanwhile they just sold more than 1 million copies of Freshwomen - Season 1 on Steam. Even if they sold all those with a 50% discount there, that would still mean around $6M. We're talking about a 170 months of CURRENT Patreon money and it's just Season 1. They also only released it on Steam in August 2022 so 16 months after it's initial Patreon release. And not all the people bought the game on a sale, with full price it'd be more like $13M so the truth is somewhere in between. The game is being developed for less than 3 years and is only on Steam for less than 18 months. Can't even compare the two platforms.
On Patreon one has to intentionally search for a game or a dev to become a supporter. On Steam, the game will reach hundreds of thousands of people just by it's recommendation system. I've found out about AVNs on Steam by accident, wouldn't even know the genre existed if not for that platform yet I bought many games and have about another 40+ on my wishlist.
If I could give one advice to any devs, especially to my favoorite ones, I'd only tell them to immediately release a demo or the first few episodes of their games for free on Steam! It takes some investments and also one has to care about marketing too but it's well worth it! That way people could try it for free, wouldn't even need to bother with refunds if they didn't like it (for slow burn, story-focused games two hours of playtime sometimes isn't enough to get a good picture of the game so the refund window could easily become too small). And if they did like it, well, they most likely will buy the rest just like I do.
Even GOG is a thousand times bigger opportunity for devs than Patreon by the way.