They would need a publisher to release it on steam uncensored since you can't just use a different site to get around your own country's laws. There's a bigger issue though which I will put below.
I wouldn't use Steam as an example here. Steam's issues with japanese games are well documented at this point. For instance, they recently banned Chaos Head Noah, though that was overturned which is extremely rare when they do that. More cynical minds would say it is because Valve at the time was recently showing off the Steam Deck in Japan and it was a bad look that they had banned a game that Nintendo had no problem with. They mentioned there would be changes going forward with this kind of thing that would hopefully eliminate or reduce the chances of it happening, but they proved that it was likely a lie as when Jast asked about looking at Full Metal Daemon Muramasa, Valve said "We are not re-reviewing previously banned apps." This game was allowed just fine at GOG and has a streamer mode.
Whatever the case, Valve or somebody at Valve doesn't appear to like many japanese games as evidenced by how many of them get banned or rejected despite being within the terms of service, or even outright safe for work versions safe to stream on youtube or twitch. They banned the SFW version of Bokuten because the publisher forgot one or two images in the files. These images were not accessible without specific tools. And of course, many japanese publishers outright don't even try to release a game uncensored, because that increases the chance of getting rejected or banned. And I repeat that rejections or bans are very rarely overturned. So it's no wonder why so many publishers don't take a chance and just put out a patch on their site for a sfw steam version (and even then this can cause the game to get banned, and publishers aren't allowed to even mention it on the store or forums).
My point is, Steam isn't the silver bullet people want it to be in this situation. Dlis would need a publisher to publish the uncensored version, and there's always the chance that it gets rejected or banned somewhere down the line. That said, it's likely that there will be at least a partial uncensor patch unofficially since a good chunk of what is in version 0.54 is uncensored. From what I remembered, all of the assets were originally uncensored, but at some point in the last couple years they began censoring new assets. It's likely what tripped DLSite was seeing the older assets that were uncensored. It sucks that Dlsite didn't want it as is, but I have heard of incidents where publishers and distributors got in trouble for selling works that weren't considered to be censored enough.
Japanese lawmakers are largely dinosaurs at this point who don't really see the need or desire to reduce or remove the censorship laws. And because it was pretty easy to just remove the mosaic for us, it would have been just as easy for Japanese people to use those tools and remove the censorship. I assume DLSite doesn't want to look like they are just skirting around the law, otherwise pretty much any Unity game could have this done and basically be sold more or less uncensored if you know how to patch it. The good news is that these laws likely won't be there forever as there are more and more younger politicians that came from manga/anime/game creating backgrounds that have pushed to have these laws removed or at least reduced. It will just take time because Japan seems to move at a snails pace when it comes to policy changes.
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