Yes and no it’s easier. For personal vehicles, the older diesels didn’t have the environmental stuff the new diesels have so older diesels are 95% visual inspection. I have a 99 diesel truck and inspection goes: pop the hood and look for obvious aftermarket parts on the engine, check for signs of tuning equipment, check to see how many catalytic converters the vehicle had when built and verify it matches, do a 3 rev pedal test to check for black smoke and dissipation rate. That’s it.

Newer diesels are more intimidate. They scan all your environmental stuff and make sure it’s working and has sufficient history of data. They scan the ECU to see if it has been recently cleared or overwritten. This is the biggest change has it can take up to a 1,000 miles to reset the ECU if you had a tune and then wiped it for smog inspection.

For commercial diesels CA says no and won’t renew your registration if the vehicle is too old. And if you had it registered commercial and want to make it personal it’s a hassle. Lots of people just sold vehicles to Mexico when this started as it was easier to buy a new commercial vehicle then go through the process.