The blocking isn't at the DNS level, you can still get to the Steam website. Either you're doing something else that changes the geolocation Steam sees you as (like a VPN), or maybe you just haven't noticed yet because you haven't looked for any games rated R18+.

The Witcher 3 is rated R18+, so you can try going to its listing page: https://store.steampowered.com/app/292030/The_Witcher_3_Wild_Hunt__Complete_Edition/