You can still turn it off. In Content Restrictions there is a drop down to limit to the old Siri. There is also a prompt, if you try to turn off Siri completely

You can still turn it off. In Content Restrictions there is a drop down to limit to the old Siri. There is also a prompt, if you try to turn off Siri completely, to revert to the old Siri. Then that setting applies to the Watch too.