Steam users in the UK who want to access "mature content games" or related community hubs are now required to have a valid credit card associated with their account. The new requirement was implemented to ensure Valve's compliance with the country's Online Safety Act, which mandates age checks in order to gain access to adult-oriented online content.
Registering a credit card is the only way to access adult content on Steam from within the UK. It's presumably more foolproof that the selfie system that proved susceptible to Death Stranding's photo mode, but Valve said going this route also "preserves the maximum degree of user privacy." This is a major point of concern: As PC Gamer's Jacob Ridley wrote last month, the new age verification requirement is a potential "privacy nightmare."
