I think, and don't quote me, there have been some changes mostly to the exhaust/tuning side of things to cut emissions. I ride a '17 Bolt r-spec, and pretty much the most common mod is to change the air intake, pipes, and a fuel controller because the stock ones are kinda wimpy for emissions concerns. That said, a cursory search seems to indicate that bikes are terrible. Of course, you have to take into account that bikes produce less emissions, however pound for pound seem to produce significantly worse emissions. FTA:

The [BMW] GS highway CO2 equivalent is a stunning 380 g/mile (17% worse than the RAM truck). They found that a 1993 Honda Shadow VX600 with only 583 ccs spews a whopping 408 g/mile. That is twice as much as a new Honda Civic.

Other studies would suggest the problem is even worse. Global MRV tested out its portable emissions equipment in 2011 comparing 12 motorcycles to 12 cars of varying years — this was featured on an episode of “Mythbusters.”. Motorcycles were almost universally terrible, with motorbikes from the 2000s producing 3,220% more NOx and 8,065% more CO2 than cars of the same era.

Not great. It seems though that based on the article, there are relatively few studies by comparison and that bikes aren't regulated near the degree that cars are. I'll also say that in the above example of a '93 Shadow, that is a carbeurated bike and in that era would have been doing basically nothing to try to curb emissions. Comparatively, a new Honda Civic is going to be fuel injected with a catalytic converter and so forth. The other point of comparison they use is the above BMW 1150 GS, which is cited from a 2008 study, so at newest a 2008 bike, which they compare to a 2020 Dodge Ram. These just aren't particularly useful comparisons because especially in the last 5-10 years, emissions standards especially for cars are ridiculously different than the era of those bikes. I would really be curious to see how something even slightly modern (like, say, my Bolt with the stock tuning/catalytic converter, etc) compares.