The basic landing has no 90 degree restriction. I have a feeling that this myth started out as one guy just making it up on the fly, and people just ran with it.
When doing the basic landing, the most important part is that both feet and hands cushion the fall.
Preferably, the first thing to hit the ground, should be the balls of your feet (near the toes), while your legs are still stretched out, but not locked. This way, the ankle is the first joint to take the pressure, then when your heels hit, your knees will start to bend, shortly followed by your waist.
At this point, you should be bent forward (because of the bend at the waist) with your arms outstretched, ready to cushion the landing even more.
As a side note, the basic landing is often followed by a run. The 90 degree "rule" will help you start running quicker, as you'll spend less time getting back up. The slowest part of squatting back up, is at the lowest point. But this is something that will come naturally as you practice. And your hands can also help you accelerate, as they are also touching the ground.