I admit that I really adore the nextcloud application and I am using it extensively for my private backups. I use Slackware Linux.
Typically Slackware users compile the source code and bundle them in packages. Slackbuild shell scripts are normally employed. They are usually found on https://slackbuilds.org or other trusted repositories.
I drafted a SlackBuild for nextcloud-client. It is available here:
I am not in favor of packaging the server application, but as slackbuilds are just shell scripts and include terse but adequate instructions on how to set it up, it is trivial to amend the scripts to exclude the packaging. Almost all Slackers rsync these scripts on their machines, so now nextcloud will be just a few keystrokes away on Slackware.
the slackbuild is normally for Nextcloud BUT the slackbuild use Owncloud as i wrote in my quote of slackbuild :
This is based on the owncloud-client slackbuild in https://slackbuilds.org