Eternum uses a mix of stuff licensed under Creative Commons and songs owned and licensed by third-party sites.
The former are safe, even if the copyright holders were to disapprove of their music being used in a game with adult content. That's the nature of Creative Commons - the restrictions they come with are limited, the licenses are perpetual and can't be revoked.
Now, even so, a copyright holder could
ask him to remove their song(s), which is what happened to DPC with The Friday Prophets, but he wouldn't be obligated to comply. Doing so would be discretionary and a gesture of goodwill. Any claim of copyright infringement could be easily contested and dismissed as made in bad faith.
The one exception to this would be if the Creative Commons stuff that's in the game was licensed under one of the NC (Non-Commercial) licenses. In that case, it would have to be removed as otherwise Caribdis would be making himself liable to a DMCA takedown.
As for the music acquired from music libraries, it depends on where he got it from. I do know for a fact that he got at least some of the songs from Epidemic Sound and Artlist. Examples:
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- I heard it in the game, liked it, looked it up and saw where it's from.
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- yes, Luna's song, as I like to call it, comes from here, unfortunately.
As soon as I found out that Caribdis is getting music from Epidemic, I sent him a DM where I told him of all the problems Ocean's had with them and cautioned against using them and a few other sites and suggested an alternative that's been safe for others to use so far. Premiumbeat and Jamendo Licensing haven't caused anyone problems, as far as I know (though they do have that pesky obscenity clause in their TOS, whose interpretation is very subjective).
As for Artlist, it caused Drifty a lot of grief and soured Leap of Faith's otherwise successful Steam release.
The short of it is that their TOS prohibit the use of their content in pornographic material. For long-term security, every song sourced from them would need to be removed, unfortunately. Even if Eternum can't be released on Steam, a DMCA takedown could still be filed directly with Patreon.
Those sites don't go actively looking for instances of people misusing their content. I reckon how they find out is because of people leaving comments on YouTube music videos ("Eternum brought me here!" and such) which might cause them or the artist of the song to see what it's all about... and if they don't like what they see, that's when the problems start.
Negotiating and securing licenses directly from the musicians is the safest way, even if it's quite a bit of hassle.