about 2 years ago - Shurenai - Direct link
It's planned for just before release or post release. Implementing it too early just bogs down the workshop with 95% unusable mods and makes life and compatibility hell for end users.

Plus, complaints for bugginess invariably come back at the devs despite people modding their experience; And currently the devs need accurate bug reports for the vanilla game, not reports from user#14515 who decided to add a bunch of workshop content and found it crashing repeatedly but doesn't actually admit they have workshop content until 4 days into the investigation.