Ok, through search on the forums I see reference to it here: Global page decorations in V2 - #4 by zef (supporting pageDecorations as described there is the goal of the successor to this API).
I also see that @ChenZhu-Xie is using it here: ABC - Accurate 🖇️ Bi-directional 🖱️ Cursor-level linking system but I’m not sure if this is something you’re still actively using or relying on?