OK, so I looked into the new situation that the OP seemed to be referring to. The brand new Brooklyn eruv is a totally different situation than the old ones, as the OP pointed it. It is a WAY WORSE situation than the old ones.
The existing Flatbush and BP eruvin enclosed sub-communities of Brooklyn with fewer than 600k people, so they could claim to be comparable to the Queens eruv that Reb Moshe was matir (still not totally sure on why he was okay with it just for Queens). The new Brooklyn eruv surrounds the entirety of Brooklyn. Millions of people, and “excluding cars” won’t help you, live within it.
Sadly, I recognize the respect the names of some of the rabbis involved with this, but they aren’t exactly making it easy to understand their heter. 90% of the letters on their site simply talk about how great and halachically solid eruvin are in-general, which nobody is challenging. The issue of Brooklyn being a reshus harabim is beyond side-stepped. They say, and I quote:
“There is no need to elaborate on the topic of permissibility of setting up Eiruvin in big cities in general, and Brooklyn specifically, since this is something that has been discussed at great length by the Gedolei Haposkim.”
I respect these rabbis and don’t want to be throwing around accusations, but they need to explain their reasoning. The Crown Heights Badatz already made a scathing rebuke, and I assume others will follow. From my understanding, even the Manhattan eiruv claims that their heter is that it’s sectioned off into sub-compartments none of which exceed 600K people. Seeing as this new Brooklyn eiruv isn’t even claiming to have done that, this might be the single most halachically problematic eiruv ever built.
If it is only kosher according to the unique shittah of the Aruch Hashulchan (which they aren’t even claiming), then that still doesn’t help with my last statement. The precedent is not to hold by it. The OU has a page where they mention nobody allows us to hold by that l’kulah. The OU.