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.
If by “regular power button menu” you mean holding the power button for a couple seconds, that was changed at least on pixel devices to bring up some bullshit called “Gemini”, some AI from googie that I never got a chance to say no to. Power + volume up is now how we get to the power menu, because of course they would change the function everyone uses occasionally to a more obscure combination without notice
