Hi Grant,
> Is there any updated information on this? Has the beta release happened as anticipated? If not, when will we see it?
I tweeted about this on April 9, Derek Law on Twitter: "Video: What's New in #WebAppBuilder for #ArcGIS and Experience Builder techni...
A beta has not been released yet. Please watch this tech session video from the Esri 2019 Dev Summit,
Web AppBuilder for ArcGIS: What's New and What's Coming - YouTube
Specifically, watch from 15:36, where the presenters introduce and preview Experience Builder (the next generation Web AppBuilder).
Hope this helps,
https://twitter.com/GIS_Bandit/status/1115854175026520064