What are the future plans for Web AppBuilder?
It's based on Javascript API 3.x for 2D maps.
ArcGIS API for Javascript 3.x has been in "Extended Support" since v.3.38 (sept 2021).
Earlier post on same topic:
https://community.esri.com/t5/arcgis-web-appbuilder-questions/web-appbuilder-end-of-life-august-2022...
Regards,
Endre