Is engine braking less effective on fuel-injected cars?

No the fuel injection is not a factor. Most are programmed to cut fuel if you lift off the accelerator with over about 1500 rpm showing and so give the best possible braking as no fuel is providing power.

In the older cars the gearing in 2nd may have been different.

What has changed over the years is a reduction in rolling resistance particularly of tyres to make cars more economical to use.

In my experience diesels are slightly worse as the fuel is not completely cut off when you lift of the loud pedal

Whether your vehicle's fuel injected or carbureted doesn't affect the engine braking. What matters is the compression ratio and the size of the engine relative to vehicle weight. Gearing also has some effect, but with a manual transmission you control what gear you're in, not the dumb box.

diesels engines do not provide much braking, and even less vacuum, hence the fitting of a vacuum pump to assist braking, even though their compression ratio is nearly three times that of a petrol engine, it won't matter to a petrol engine whether its carburettor or fuel injection, the engine resistance will be the same going downhill.

It Could be the fuel you are using.

Have you tryed using this type of fuel which is FREE

You can run you car for free on water

Pretty amazing stuff here

No, older cars were generally lower geared, lighter & had longer stroke engines with more low down torque than modern short stroke multi valve units.

normally, the brake system will work well as long as it is not worn out on the car. However, the necessary parts may differ that are needed for that type of car. the thing is, that would only mean different parts, it may not necessarily mean more effective or stronger parts.