Which version of openssh support IPv6?

Openssh supports IPv6 since 2000. Use the latest version to avoid security bugs.

How can I enable/disable IPv6 in ssh daemon?

There is a configuration option in sshd_config file to define the local addresses sshd should listen on.

ListenAddress

Specifies the local addresses sshd should listen on. The following forms may be used:

                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port

How can I enable/disable IPv6 in ssh client (ssh/scp/sftp)?

There is a configration option in ssh_config file to define which protocol (ipv4/ipv6) enable/disable:

AddressFamily

Specifies which address family should be used by the ssh client programs. Valid arguments are any, inet (use IPv4 only), or inet6 (use IPv6 only). The default is any.

Is there any other way to enable/disable IPv6 without modifying configuration files?

You can force IPv4 or IPv6 usage with commmand line switches -4 and -6 respectively.