It appears that access to the Steam Community, including forums, user profile pages, and inventories, has been blocked in China. According to Greatfire.org, a site that monitors online censorship in the country, access to steamcommunity.com has actually been 96 percent blocked since December 15.
Store.steampowered.com remains available, meaning that users in China can still purchase and play games. But anything that requires interaction with others is off the table. Why the ban was put into place isn't clear, but one possible motivating factor is the changing face of gaming, and the gaming industry, in China.
Tencent, which despite its massive size remains not a household name outside its home country, recently indicated that it was gearing up for a worldwide breakout, but at the same time the number of Steam users in China has absolutely skyrocketed. Much of that increase comes courtesy of PUBG, a game which not too long ago was also facing a ban—until Tencent signed a deal to officially publish the game in the country, in accordance with "socialist core values." A previous effort by Tencent to acquire PUBG publisher Bluehole had been rejected.
Chinese authorities have a long and well-established history of acting arbitrarily to ensure advantages for Chinese businesses, and this would certainly fit that pattern. But it could also be a very straightforward crackdown on assembly and speech, too—another familiar feature of the Chinese political landscape. I've reached out to Valve to find out more about what's going on and will update if I receive a reply.