A post on X by MellowOnline1 from Sunday reports that user details for approximately 89 million Steam accounts are being offered for sale on the Dark Web following an alleged data breach (thanks XDA). A second post follows up with indications that the unconfirmed leak was through a third-party vendor, rather than Steam itself. As said, this is unconfirmed, but the included suggestion that users change their passwords seems like an easy way to be prudent as this gets sorted. Here's a bit from Sunday's post:
Yesterday, an alleged major @Steam data breach occurred, compromising over 89 million user records (roughly two-thirds of all Steam accounts).
These datasets are being sold for over $5,000 on what appears to be a site akin to Mipped.