Steam isn't officially available in China, but it's not officially blocked either. But this inbetween state still gives the censors unofficial power to ensure that Steam does not allow adult games to be sold in China.
Steam only recently stopped censoring adult games in the rest of the world but the change of policy will not apply to China.
As part of the policy shift, steam added two more content filtering options for users: A general Mature Content filter and an Adult Only filter. But China doesn't have the latter option, which means that they don't have access to these games at
Chinese media speculated that Steam is restricting adult titles from Chinese gamers to avoid getting officially blocked in the country. China's government is tightening its grip on the gaming industry and repeatedly clamps down on online content
that they deem inappropriate, so Steam could be trying to keep a delicate balance: Not officially blocked, but not officially banned, either.