What VintageMopar said is a large part of the reason I prefer older vehicles and trucks. (You'd better have your wallet handy if you want me to work on anything newer than 1980 ... sex will also work but only for my wife.)
Leaded gasoline aside, the main cause -- by a wide margin -- of emission, pollution, and smog is neglected or non-existent maintenance.
To deal with people failing to maintain their vehicles, Congress, state legislators, and administrative agencies came up huge pile of regulations. Relatively new vehicles started failing emissions tests so people blamed the manufacturers, not realizing their voting and failure to adequately maintain their vehicles was the real cause.
The manufacturers added doodads to address government regulations, owners' failures, and (most important to manufacturers) falling sales. The doodads sometimes worked when new and regularly maintained. (In case you didn't notice, we're right back where we started, without addressing the actual problem.)
The fact most emissions inspections and testing require "as manufactured" arguably gets in the way and drives up maintenance costs. Even if a 'stripped down' vehicle produces less emissions, it can't be registered in many places because it's not an OEM configuration.
So, we're left with a vicious cycle of good intentions making things worse.
Personally, any engineer who doesn't adhere to K.I.S.S. -- Keep It Simple, Stupid -- should be horsewhipped.
