Steps to reproduce:

1. Set kernel command-line parameter ""ipv6.disable=1", update GRUB config and reboot
   Example:

1. 1. Edit file "/etc/default/grub"
   2. Append " ipv6.disable=1" to the "GRUB_CMDLINE_LINUX" parameter.
   3. Update GRUB boot config:
      # /usr/sbin/grub2-mkconfig -o /boot/grub2/grub.cfg
   4. reboot
2. Verify that the file "/proc/net/tcp6" no longer exists:
   $ ls /proc/net/tcp6
   ls: cannot access '/proc/net/tcp6': No such file or directory
3. From Linux CLI, test item key:
   $ zabbix_agentd -t 'net.tcp.socket.count[,,,,established]'

Tested on:

Rocky Linux 8 (4.18.0-513.11.1.el8_9.x86_64)
Ubunt 22.04.4 (5.15.0-101-generic)
Fedora Workstation 39 (6.8.6-200.fc39.x86_64){}

Result:

Response returned is:
net.tcp.socket.count[,,,,established]         [m|ZBX_NOTSUPPORTED] [Cannot open /proc/net/tcp6: [2] No such file or directory]

Expected:

Response should return the number of ESTABLISHED TCP connections. Zabbix Agent should be able to return a result with or without IPv6 enabled.