It's a silly topic... but Eagle doesn't/didn't support plated slotted holes.
Most PCB manufacturers support the workaround of drawing a milling line (milling diameter = line width) in the milling layer, wich will later become the plated slot.
By generating the Gerber file, the milling layer is exported to the dimension-gerber. Generating the slots in the drill file (*.xln) is not supported.
in Zofz:
Wish & possible Eagle-Gerber-Export:
Eagle & slotted pads ...
Re: Eagle & slotted pads ...
Cadence/allegro, the same problem
Re: Eagle & slotted pads ...
I am still thinking how to solve this. What comes to my mind is to treat loops as the board outline (also non-plated) and the thick open lines as rout plated features. How error-prone would that be? You know, there are strange things on the board outline layers, like text or dimensions arrows, sometimes.