Amazingly, modern chains are narrower on the outside, but effectively wider on the inside, which widens the "usable range" of friction shift lever position for each gear selection.

As well, the beveling and bulging of the outer sideplates means newer chains are more aggressive at engaging the cog's teeth, which makes for the ability to transmit more power even during a shift.

Modern chain's inner spaces are thus shaped to "funnel" the tips of sprocket teeth into engagement, while a modern, narrower chain is also less prone to rubbing against the next-larger cog than it is running on.

These newer chain's greater flexibility reduces lubrication requirements while also allowing the chain to better engage two cogs at once during shifts.

The newer chains generally seem to last many thousands of miles with scant lubrication, are much lighter, and are strong enough for hard-core mountain biking using even a 22t chainring.

With C&V bikes, I am often trying to install the narrowest chain that I can use on a given bike's crankset, and usually 9s chain works fine.
I prefer 9s chain on all Varsity/Supersport steel cranksets, and even on most cottered cranksets.
But older Stronglight alloy cranksets (and SR clones of such) may need tooth reworking (bending/beveling) to even use 7-8s chain reliably without "skating" after a downshift.

9s chain seems to give the best shifting performance on all Suntour and Uniglide freewheels, fwiw.

I think that metallurgy/processing advancements have been the key to these narrower chain's ability to remain durable and wear-resistant despite much less metal on board.

Modern chain has transformed friction shifting and the vintage-bike riding experience for me, and has made all of the mechanical work (setup, adjustments, cleaning/lubing) a bit easier as well.
During a recent gearing upgrade to my 1991 Torelli, I replaced the 7s Regina Synchro-spaced freewheel with a newer, larger, SIS-spaced Sunrace 7s freewheel.
But only after installing narrower 9s chain could I overcome the incompatibility between the Synchro-7 cog spacing and the Sunrace/Shimano-7 cog spacing. Wow.

Any reason why man or woman would have particular difficulty understanding these here observations?


Edit: The 9s SRAM Power Links and inexpensive KMC 9s Missing Links seem to both do a good job of connecting Shimano 9s chain.
The SRAM 10s Power Link fits too tightly on 10s Shimano chain as I found out the hard way.
KMC makes two widths of Missing Link for 8s chain. The narrower "Missing Link II" model is too narrow for Shimano 8s chain.