It appears Valve is applying a case-by-case approach.
Recently, several developers began claiming that Steam has supposedly banned adult games from launching in Early Access. For instance, Dammitbird reported this.
One developer reached out to Steam Support directly — the representative rejected the reports of a blanket “ban” and said each title is evaluated individually:
“We assess each game on its own merits when reviewing its build and store page. There is no single rule that bars certain games from using the Early Access model for development.”
Notably, this reply contradicts what Steam Support reportedly stated a month ago:
We do not distribute Adult Only titles in Early Access. Sorry. You may update the game after its full release, but if you plan to add additional adult content, we ask that you provide it as a (potentially free) DLC.
Source: iXBT.games