By default on the dedicated servers disabled Caves maps and Entities radar.
It can be enabled via send to players a system (from the server) message with color codes:

I don't quite understand why did you make this opt-in instead of opt-out.

Some arguments:

• even if the features are allowed per server rules, this mod is currently small enough that many admins wouldn't bother to allow it
• per previous argument, admins may not know how to properly enable this or may not be able to (specific plugins overriding something, etc.)
• the enforcement of the rules is still up to server admins regardless of this setting, in other words the user may just use a hack mod or such to enable a similar feature

When I started working on the map, I primarily focused on one popular Russian server of minecraft. On this server, rules prohibit the display of caves and entities on the map. Therefore by default I did it to hide. And I don't want to change this setting. Most servers use a welcome message when you log on to the server, it's not difficult to add to the end of that message the combination of color codes.