The Classic Offensive Website now displays the hashtag #AllowClassicOffensive along with a message saying "Valve has requested we do not distribute Classic Offensive," a mod intended to recreate the gameplay from Counter-Strike: Global Offensive (thanks Reddit). This links to a timeline showing that the run-up to launching the mod was rocky, as Steam's guidelines for allowable mods shifted along the way, despite the team being told its project would be allowed as far back as 2017. This is discussed in the Classic Offensive FAQ which raises questions about whether Valve's overall stance on modding its games has changed:

By further digging into Valve's statement: the past three decades of Valve modding, and Valve's stance on modding with the recently updated Distributing Source Engine Games / Mods guidelines, shows that they are now against general modding of their games.

While they have licenses for using some of their tools like Hammer, modding as a whole only applies to a very small list of their games and must solely be done using Source SDK 2013:
- Half-Life 2
- Team Fortress 2
Valve can at any time take action on any unlicensed mod or "skin" for any reason, this includes the Portal, Counter-Strike, Day of Defeat, Left 4 Dead franchises, despite their history with modding and various websites distributing mods like GameBanana, ModDB, Nexus Mods and more.