The support launched alongside Terraria's latest patch today: version 1.4.2. The patch doesn't include any other improvements or content additions of note (aside bugfixes), so it's safe to assume the game's Workshop implementation has occupied most of the developers' time lately.
https://www.techspot.com/news/89099-terraria-receives-support-steam-workshop-mods-over-10.html?utm_source=dlvr.it&utm_medium=blogger
https://www.techspot.com/news/89099-terraria-receives-support-steam-workshop-mods-over-10.html?utm_source=dlvr.it&utm_medium=blogger