How to create isolation slots/gaps in eagle CAD
I don't think this is specific to Eagle, but generally if you want milled isolation slots you will specify that on a mechanical layer, the same as the outline routing or V-grooves. 

Typically (check your PCB manufacturer's instructions) slots that touch copper will be plated by default and if they don't touch copper they'll be unplated by default but you should be able to change that with a call-out on the mechanical layer. 

The center of the drawn contours should be the edge of the milled slot- in other words the width of the lines you draw will not be used in any way. 

Note that the manufacturer has a choice of tools to use to do the milling. Inside corners will, by necessity, have a non-zero radius (equal to the radius of the milling tool), so even if you draw the corners sharp they will end up with some internal radius. You can ask for a specific tool radius but they'll probably use what they want. For example, if they use a 1mm diameter routing tool you'll end up with 0.5mm radius on internal corners (and, of course, your minimum slot width will be 1mm). Larger tools will cut faster, wear slower and be less prone to breakage, so they are going to tend to prefer larger radii. If you design the slots with rounded internal corners with radii equal to half the slot width you should not have any unpleasant surprises on that account. In practice that may mean making them a bit longer than you would otherwise make them.