Maybe I've found out the reason why I don't have that option, despite having turned on Dark Mode.
It's because it's not the system-wide Dark Mode like you said, it's the "half" version that can be activated with Onyx.
It's more subtle as it only darkens the Dock/Menubar/Notification Center, but not the windows/applications.
I turned it on long ago as I love Dark Mode only in those places, then forgot about it all.
Since it's a half version, the system doesn't recognise it as true Dark Mode obviously.
So I don't get options like that one in Maps, since Maps remains Light to begin with.
Actually kinda stupid of me to expect that option, when I don't really need it anyways.
Also now I'm stuck with the subtle version, I can't get back to the native system-wide one.
Onyx is not yet available for Catalina, and I've already upgraded to Catalina.
I've tried switching to Light then Dark again in System Preferences > General...
Light goes full-on Light, but Dark remains subtle so I suppose I must disable it within Onyx.
Or is there a native way of resetting the half Dark Mode to the full version?
It's not a real issue, I can wait for the release of Onyx for Catalina...I'm just wondering.