https://nextcloud.com/blog/open-source-email-pioneer-roundcube-comes-aboard-nextcloud/
Becoming a new steward for Roundcube, Nextcloud sets sail to foster the growth and improvement of the project.
RIP Roundcube, farewell my friend. So, what should we expect now? To have RC as NextCloud's default e-mail interface OR to get RC filled with mindless bugs and crappy features/decisions? Read this to see how bad NC's Webmail is: https://lemmy.world/comment/5490189
Also, what about Kolab https://kolab.org / https://kolabnow.com? Besides providing e-mail hosting they seem to be the ones pushing the development of RC and essential plugins that are somehow competition for NextCloud.
Side note, in 2006 Kolab Systems raised more than $100k USD to develop "RoundCube Next" as a next-generation mail and communication platform, there is little to show for it and no active development.