Hi folks! Hopefully I can give some useful info here, it’s an important topic for me 
Yes, the Spline system got pushed. It was a bummer for us as well - but we realized that before adding new tools and functionality, we absolutely needed to build up a better tooling foundation.
So, we’ve been hard at work on this, with Splines coming soon after, we hope:
More info:
Unity - Draggable overlays, custom toolbars, new Tool systems - Unity Forum