A Phoronix reader -- and backer to RoundCube's IndieGoGo campaign that raised $103,541 of their $80k USD goal from 871 backers -- pointed out that the project remains at a stand-still. The GitHub repositories haven't seen any activity in months. The RoundCube Server hasn't been touched since last November, the mail application and client side code since October, etc.
The IGG updates area last update was in May of 2016 when Thomas Bruederli wrote that they were understaffed and needed to get a development team back together. Meanwhile, the comments page remains filled up with various negative comments from those upset over the project's apparent failure. Their Twitter also doesn't instill any confidence with it not being used since last year.
Roundcube continues to be maintained but don't hold your breath waiting for RoundCube-Next with its big rewrite, new user interface, Jabber chat support, video conferencing, scheduling, and more, as it doesn't look like this milestone will be realized in 2017 if ever.