Drill into steel frame for cargo rack So I have a bike without pegs for the mounting of a cargo bike rack.

It is a steel frame so I was thinking of installing make-shift pegs myself.

Is this going to breach the integrity of the frame?

I know some types of metals and constructions aren't intended to be drilled through.
It mostly depends on the thickness of the tube.

Chainstays and seat stays tend to be quite small tube, so drilling them takes a larger percentage of metal away.  I would not drill a stay unless I had a welder to add metal- instead I'd use a clamp-on fitting.

For downtubes and seat tubes, the tube itself tends to be wider.  I've successfully used a nutsert or rivnut to add bidon mounts to a steel road bike.  This involved careful drilling, then using a special tool to mount and crimp a special insert into the frame.