China has blocked several of Steam's services that rely on the Community part to work.
This includes the Steam Community page itself, which provides a forum for every game, and other associated features such as the user inventory and profile pages. This also extends to achievements, and really anything with social interaction.
Steam itself, and the store, remain available as of press time. Chinese players will still able to play the games they own, or buy new ones, but the hub pages for games won't show up for them. It's pretty bad, but nothing out of the ordinary for China.
As we've seen in PUBG's case, any foreign-made product that gets too big either gets replicated by local companies, forcing it out of the country, or China blocks it and only allows it back if the makers play ball. PUBG was facing a threat of being banned, until developer PUBG Corp signed a deal with Chinese conglomerate Tencent for the latter to distribute a modified version in the country.
There's also Steam's competitor in China, Tencent's WeGame.