For a Next Gen RE, the Junos OS runs as a virtual machine over a Linux based host(VMHost). In your case, if it's a Next Gen RE, then the steps to disable Magic_SysRq_key remains the same prior to 17.4 too.
As already explained, kernel.sysrq = 0 indicates that the Magic_SysRq_key is disabled. If kernel.sysrq = 1, then you can disabled it using "sysctl -w kernel.sysrq=0" but make sure you login into the VMhost using "vhclient -s" before you execute the command to disable the Magic_SysRq_key.
You receive the error "sysctl: unknown oid 'kernel.sysrq'" if you execute the command before logging into the VMhost shell.
Hope this helps.
Thanks and Regards, Pradeep Kumar Technical Support Engineer | Juniper Networks *: +1-888-314-5822 *: firstname.lastname@example.org