too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
It isn't that the chains are weaker, but that they wear out faster. There has been slight thinning of the plates, but that's offset by changes in the steel used and heat treatment, so the breaking strength stays above anything riders can need.
The issue is that narrowing the chain, reduces the contact area in the bearing (pin) increasing contact pressure and wear.
So the wear life will be shorter, but breakage is still mainly because mechanic or rider error, specifically bad splicing and/or side stresses from aggressive shifting under load, same as with 8s.
Pretty hard to confirm that 9 - 10 - 11 speed chains actually wear "faster" - my experience has been that the 9 and 10 speed Wipperman chains are very durable. HOWEVER these drive systems are much more sensitive to chain wear (both stretch and slop) than the earlier lever shifted, non index gear systems. Sloppy shifting and noise are symptoms of chain wear.
You might expect to get 5000 miles of good use from a chain used in an old school driveline, where a 10 speed chain with 2500 miles on it is a candidate for replacement. All things being equal, a dirty chain will wear "faster". Keeping your chain clean and well - lubricated really helps get the most out of them.
In current derailleur chain, the punching the hole in the inner side plate
provides the metal that supports the roller, its pushed in.
hot forged, I expect,
that edge contact is where the wear is concentrated, contacting just the ends of the roller.
and the inside has the pin passing thru it, held in place in the outer side plate..
if the side plates are thinner that bushing ring is thinner too..
older full bushing chain uses a separate sleeve in the inner link
it supports the roller an in turn is supported by the pin, full width.
not as laterally flexible, but was the [Regina Oro etc] 3/32 6 speed
derailleur chain in the 60's, and track chain in current use. 1/8"