The idea of a Steam release, as is the case in basically every other non-Steam game, has been floated around endlessly for the past however many years. However, much of what is said seems to be speculation, that the game might be delisted or banned or that Valve would somehow require some sort of content control for the game. Therefore in this post I will try to clear some of this up, so that the idea can be discussed in a slightly more informed way, or that, perhaps, other people here can share ideas & examples of games that may have been removed for similar reasons.
1. The appropriateness of Haven's content
It has been my experience (I am a lazy man and will not attach examples, apologies if I am bullshitting) that whenever the idea is brought up this is one of the first concerns people have. However, I do not think current Steam policy warrants such a concern. Their stated policy, as far as I can tell is that
With that principle in mind, we've decided that the right approach is to allow everything onto the Steam Store, except for things that we decide are illegal, or straight up trolling. (steamcommunity dot com/games/593110/announcements/detail/1666776116200553082?l=english) (apologies for bad link, new account)
The quote is from a post from 2018 and I can't seem to find any more, but I think if you look at the games on Steam as of now I think this is probably still their policy:

etc. There are some more specific things here (partner dot steamgames dot com/steamdirect), but none of them as far as I can see apply more to Haven than many other unmoderated multiplayer games which are on there without a hitch.
If you are worried that the game might be banned based on its forum/in-game player moderation, I believe there are other games that have been on steam with many years with also essentially none (if one is looking for an example with a community of similarly infamous reputation, Blockland was also off steam for many years before such a transition that led to no issues at all with this AFAIK). If the worry is for user-generated content on the Steam platform itself, I doubt it would be much trouble for jorbtar and the forumers to just assign the same forum mods for the inevitably dead steam forums and to deal with the windmills on screenshots and such (or let Valve take care of it, since that's an option). There are sites that track de-listed games from steam (see steam-tracker dot com), and as far as I can tell almost all of the removals are copyright, redundancy, legal stuff etc. Closest thing I can find to a game that has been removed for Haven-plausible reasons is this gamedeveloper dot com/business/domina-pulled-from-steam-after-dev-posts-transphobic-tirade,which seems implausible for Haven still unless someone goes deeper into the deep end than previously imagined.
2. Technical annoyances
Issues which might be more salient are maybe the issues that Runescape's Steam Release had, IE custom clients and multiple instances requiring tricks. I say tricks instead of "not working" because both issues, which are admittedly annoying, have (as far as I can tell - maybe someone who knows more about Runescape can comment here) pretty simple player fixes. For Runescape, it seems that they made it so a Steam account can only link to one account (?) which might be the source of their issues, so maybe this is easier to avoid than it looks.
Also, I think you'd have to (finally) wrap the game on an exe file for it to be on Steam, or apply some other shenanigan (reddit dot com /r/SteamDeck/comments/145yt08/does_anyone_know_how_to_add_java_games_to_steam/). Custom clients might need to apply the same shenanigan if they want players to be able to do the aforementioned Steam hour tracking (for those who care about this sort of thing), but I would be surprised if it was that annoying. I am almost sure Steam has its own way of handling games with pre-install requirements to handle the whole Java situation, though, but I could be wrong.
3. The review issue
Many others have also said that the game has a high chance to review poorly on Steam. All reasonable men agree. On the other hand, I don't think the devs care, and I don't think users would care all much either. Plenty of Steam games have thousands of concurrent players and awful review scores. As far as I can tell, the game nowadays mostly gets new users from word of mouth/friend recommendations (for which the Steam score doesn't really matter, and for which Steam might help, given how often my friends are annoyed by having to download some game somewhere weird) and the odd internet content straggler (and for this one the score might matter, but I doubt all that much). Other games that have previously been on Steam have had new player influxes because of it, sometimes moderately small and sometimes quite sizeable.
4. Other issues I have heard of before
It has been believed that Steam's cut of profits would probably turn jorbtar off. This might be an actual big issue; Valve takes 30% of all IAPs, as far as I can tell, indiscriminately (partner dot steamgames dot com/doc/features/microtransactions). However, again with only the limited vision of my own, that when it comes to these things there might be some nuance at play under NDAs and such (I doubt Jagex is forking over 30% their profits for the majority of their playerbase which is off-Steam). I think it is best not to speculate much on these things unless there is solid information to offer. It would be interesting if others had examples here.
Lastly, which is I think the actual key question, is that of hassle. There's a lot of factors at play here, as you can see - it's not just a matter of posting the thing on Steam and having it be there, and it's hard to gauge the potential benefits from here. There are games that have inexplicably exploded after being put on Steam despite years of being apparently easily available on their website (a big example is the aforementioned Blockland), while for some games it doesn't seem to have changed things all that much (as is the case of Runescape as far as I know. Forgive me for the tiredness of these 2 examples, but I do not know many games which fit this criteria. People who know Transformice and old Maple Story might have more informed opinions on games which were off-steam going to it).
Is this worth all the effort? I don't know. Personally, given the personalities of our developers, I think they very highly value simply not being hassled over potentially needless capital b Bureau complications over the chance of this helping bring some, I think we all agree, much-needed new blood to the game, so the chances are low of it happening. If it does, I think the odds are good that it ends up being mostly the same plus some very minor annoyances for Steam to keep track of clients and such.
On a last note, I don't think one can have a very informed opinion on how much the game's population would chance because of such a thing. It certainly won't decrease, and every new source of players is a good thing, but I would be surprised if the change was as massive as some hope. If I were jorbtar, I would probably put this off until some new world/big announcement situation, and, even then, reconsider thrice more. It could be a bunch of hassle for nothing, or it could be a nice little stream of new players.
I hope this is of some use to somebody, since this topic seems to show up so often.