I’m not aware that SmartThings ever teased any such thing. When @ady624 (a SmartThings employee at the time) last made a presentation on the topic some years ago, he made it clear that building a webCoRE-like interface on top of the Rules API was something that would have to be done by the community.
Unfortunately, many users interpreted this to mean something very different and false information spread in various places online, resulting in unrealistic expectations and unnecessary hand-wringing by webCoRE users.
A former webCoRE (ab)user myself, I posted elsewhere about my own transition and views on the alternatives, so I’m not going to repeat that here. While I’m still using webCoRE a little for a couple of pistons I haven’t yet been able to replicate in the Rules API, in general I agree with @orangebucket: It’s time to move on or move over (to Hubitat).