So this integration is a part of the client, both for Nautilus and Dolphin and thus should be in the deb packages. Packagers might split it up or not ship part of it, that is up to them of course.
I posted here on the status of Linux distribution packages: Linux packages status
I see it links to packages here: http://download.opensuse.org/repositories/home:/ivaradi//Debian_8.0/
And I see Nautilus integration but not Dolphin. This is packaged by “ivaradi”, using the Open Build Service, the project page is here: https://build.opensuse.org/package/show/home:ivaradi/nextcloud-client
I suggest to ask him if he can add the Dolphin package or help him do that. Well, ok, I just asked him, see here: Linux packages status
At least I think it likely that it is the same @ivaradi 