Do trucks still have mechanical spring parking brakes?A parking brake isn't a secondary braking system on anything modern with a split brake system.
On a modern vehicle, you should never completely lose braking with no warning, and if you do, then something major has gone wrong.
However electric handbrakes can still be applied in an emergency. They may not engage immediately, but they will still apply.
And if you're that concerned about modern technology and braking, you might not like to know that pretty much all modern trucks, the first 80% of brake pedal travel is pure electronic. Each axle usually has it's own electronic modulator to control air to each wheel.
You can cut the airlines out of the brake pedal, and the brakes will still work as long as the electronics are working, and the axle modulators have air pressure. It's only once you get past that first 80% does the air valve in the pedal open, and bypass the electronic modulators.