On my ditto (running GrapheneOS), the Lockdown option is accessible through the regular power button menu. When I press power+volumeUp it switches to silent mode. I don't know if/where I changed this, since I can't find the options when searching in settings.

EDIT: I just found it - in Settings > System > Gestures > Prevent ringing. I can either set power+volumeUp to mute the phone, or vibrate only. Nothing about the Lockdown option, but having it in the regular power button menu is good enough for me.