Nextcloud is a fork of Owncloud that's led by the original Owncloud developer, and it seems to be more popular with the free software and Linux community. It looks like both are still active.
I'm still running OwnCloud. I've tried twice to move to nextcloud but run into syncing issues from my clients with the same data (and same basic setup) that Owncloud is currently handling fine.
Long term I do still plan on moving to NextCloud, the core dev team from owncloud moved over and I've always been happy with OC, but I need the time to actually figure out why I'm getting sync errors with the suggested setup after following all the suggested fixes.