You certainly can mill a slot that width to that depth, but it is going
to take extended length cutters. Long cutters chatter, especially when
cutting around their entire perimeter in a slot. Carbide cutters work
better when dealing with difficult length to diameter ratios because
they are stiffer. You just have to be careful with carbide not to
chatter them or they loose their edge.

I might take .010" at a pass, at about 5 IPM and 2500 RPM, keeping the
slot clear of chips with air or flood coolant. If you make your cut,
then make it again shifted over a few thou in Y both ways, you will
keep the cutter from contacting the side walls as you get further down,
to avoid chatter.

You can cut hard steel this way, so long at it isn't full hard. Crappy
steel, with hard inclusions (chrome chips from the bumper it was made
from, ha!) is where I get into trouble.

I would expect it to take a while to do this, and I wouldn't be
surprised to go through a cutter or two.