Steam has existed in a grey area in China, technically not following rules which require all games be approved yet never drawing the ire of authorities. Many have long expected it to eventually be banned, though, especially after Valve partnered with Perfect World to launch the limited Steam China. This might now be happening. Maybe. Or it's possibly being hindered, at least. This all started with a big stink on Christmas Day that led some to declare Steam had been banned by the Great Firewall which restricts access to many sites outside China, but the reality is not so clear cut.
On the 25th of December, 2021, reports went round the Internet that the Steam storefront and community pages were not accessible in China. Many assumed that authorities had finally banned the global version of Steam. Some subsequent reports claimed that it was 'DNS poisoning' preventing people from accessing the store, which some people took to mean it was an attack by wrong'uns rather than censorship.
Steam is not outright down in China, mind. Access seems intermittent, sometimes not loading, sometimes loading slowly. Game Developer (the site formerly known ha-ha-hilariously as Gamasutra) relay reports that many users are able to get on Steam, they just might have to retry or wait a few minutes. You can see the inconsistency yourself with Great Firewall testing tools; while some quickly declare it's blocked (stoking belief in a ban), others with more patience might show you it loads sometimes, or after a long delay.
Game Developer also pointed to a Twitter thread by David Frank, who claimed what we're seeing is interference by misusing technology called Server Name Indication (SNI) Detection. According to his account, yes, it may well be an intentional effort to interfere with Steam, and the fact that it works intermittently might be intentional to create plausible deniablity.
Chinese authorities hardly display a public list of banned websites, so people are left guessing what's going on. They're known to use a variety of methods to block access too, as well as often interfere with sites and services rather than outright blocking them. All of which means, we don't know for certain what's going on right now. But many assume it's a first step in cracking down on Steam, attempting to push people onto the censored local version.
Steam China has been fine throughout this. It is a limited version launched by Valve with Chinese publishers Perfect World in February 2021, which only offers a handful of games that are approved by Chinese authorities. While the global Steam catalogue includes over 63,000 games, Steam China has only 53. Authorities have reportedly stopped issuing game licenses for some time too, with no games being approved since July 2021 (not just for Steam, for any games). People definitely have reasons to want Steam proper.
While precise numbers on Steam's popularity in China are elusive, you can infer a lot from the fact that Simplified Chinese is the second most popular language on Steam, after English. Valve's stats say almost 25% of users are using Simplified Chinese. That's a huge number of people facing potentially losing access to games (or forced to use workarounds like VPNs), and a big financial concern for developers.