The past few weeks have seen Debian stakeholders debating over ownCloud as an unfit upstream with it being a pain to maintain the current ownCloud Debian packages. Due to the unwillingness of the ownCloud developers, managing ownCloud upgrades being a headache, and Debian packaging rules, it's looking like the Debian developers maintaining these packages are getting ready to throw in the towel for Stretch. Of course, others are wanting ownCloud to continue to be packaged since there is no viable open-source alternative to this project.
There still would surely be third-party packages for running ownCloud on Debian, but they wouldn't be part of the official archive. You can read that aforelinked mailing list thread for all of the drama. Fedora already ended up in a similar boat with regard to packaging ownCloud.
Jos Poortvliet of ownCloud has tried appealing to the Debian developers but it's not clear yet if any compromise will be made so ownCloud can continue to be maintained as Debian packages.
Those wishing to manually install OwnCloud can visit OwnCloud.org.