And still, it was supposed to not be vulnerable to enemy radar. And a multi decade old anti-air system, with the benefit of good intelligence and incredibly sloppy American operations, managed to shoot it down.

Why would anyone think a helicopter that would be flying much lower to the ground, would be invulnerable to e.g. man portable air defence systems?

Because low flying aircraft are harder to detect than high flying aircraft. More over when their rcs has been significantly reduced. It’s not “invulnerable” no more than any stealth aircraft, submarine, tank, or any other platform is. It’s significantly harder to defeat.

> Because low flying aircraft are harder to detect than high flying aircraft

From afar. But an attack helicopter will by definition be near the battlefield/enemy, so they'll have plenty of opportunities to see it and react.

it wasn't vulnerable to enemy radar, serbian AA realized that it was the exact same pattern day in and day out, did some quick calculations, and fired at the spot it knew it would be at the next day.

and that worked.

No, they timed their radar scans so that they caught the F-117 with its bomb bay open.