I'll tell you my scenario and what worked for me.
I'll list, in chronological order, all the changes I made, because in the end it might have been a combination of changes which fixed the problem and not the final step alone.
The bike: a steel hybrid bicycle with v-brakes; single walled rim, with black anodization; new brake pads.
Rim condition: the anodization 70% worn, wear groove still deep, rim in good condition.
Here are two pictures at different angles:
Step 0 (initially):
The pads are toed in slightly as all the instructions on the internet suggest. All mounting bolts tightened using the recommended torque. Rim cleaned with isopropyl alcohol. No play in fork or wheel bearings. Very loud squeal when braking hard (but not as hard as to have the rear wheel lift off).
Step 1: Tightened the brake arms and brake pads bolts slightly more. No improvement.
Step 2: Removed the wheel, tightened the bearings slightly more (but still allowing smooth turning of the axle), even though there was no play. When I put the wheel back on, I tightened the QR skewer a bit more too. No improvement.
Step 3: I slightly sanded the rim and the pads using 600grit sandpaper, then applied a small amount of baby powder on rim and pads. No improvement.
Step 4: Added more toe in. The squeal got worse.
Step 5: Removed the toe in completely. I set the pads flat on the rim. The squeal was gone entirely.
I'm not sure why this worked on this bike. My other bikes with v-brake have the pads toed in slightly, with no squeal at all. I think there are too many variables that influence squeal, like pad material and hardness, pad shape, rim condition, brake arm flexibility, fork flexibility, temperature, humidity, spoke tension and lacing pattern, etc. But the most important one in my experience is pad angle relative to the rim.
Maybe someone will shoot a slow motion video, or do a more scientific experiment sometime that will reveal more information.