If for any reason you want to disable ipv6 and don’t want to do it with the sysctl directive, if your kernel was compiled with ipv6 support as a module one easy route is to blacklist the ipv6 kernel module.
All you need to do is create/update the ‘/etc/rc.modules.d/blacklist.conf’ file and add the line ‘blacklist ipv6’.
alternatively you can run the following command as root.
echo '
#Disabling IPv6
blacklist ipv6
' >> /etc/modprobe.d/blacklist.conf
Sources:
https://www.linuxquestions.org/questions/slackware-14/how-do-i-disable-ipv6-in-slackware-14-1-a-4175504618/
https://www.linuxquestions.org/questions/slackware-14/how-to-disable-ipv6-937447/