Feif Un,
The Letter was signed against the Flatbush Eiruv specifically and was signed by Rav Shmuel Berenbaum zt’l, Rav Aaron Shechter shlita,Rav Hillel David shlita, and Rav Feivel Cohen shlita(Mechaber of Badei HaShulachan).
Furthermore Rav Feivel Cohen headed a delegation to Rav Elyashiv shlita to present the shaila to him and he gave over the written psak of assur as was circulated at the time.
All the above Rabbonim hold the Boro Park eiruv is assur however they did not wish to sign publicly against it in order to be able to ensure that the Boro Park Rabbonim who were Pro the Eiruv would not be able to intrude on their “turf” in Flatbush. (This was stated publicly by Rav Cohen shlita in front of his entire shul).
Rav Moshe wrote clearly that Brooklyn is a Reshus Harabim D’Oraisoh to state otherwise is to simply overlook the fact that it is written in Igros Moshe.
To state that Rav Moshe “ignored” the Gedolei Achronim is basically admitting that one is a complete Am Ha’aretz.
Yes there were Eiruvin in Europe.
Europe was a lot smaller the Brooklyn is today and the need for an eiruv was overwhelming. Meaning it was needed for basic things like going to the bathroom and getting one’s food from the communal oven.
Do you follow the Mishna Berurah?
Well the Mishna Berurah himself writes that the eiruvin in Europe were established based on many Kulos and a “Ba’al Nefesh should be Machmir. He makes quite clear that he was not very happy with the stae of Eiruvin in Europe.
Did the Mishna Berura also ignore the Achronim?
There are no mainstream competent Flatbush Poskim that I am aware of that hold of the eiruv in Flatbush. It was actually built by a Rov from Monsey who is the son of one of the Gedolei HaDor and when his Father was made aware that he went against the express wishes of all the Rabbonim of the area he was furious. I cannot report the exact exchange that occured since there are conflicting reports suffice to say that his father lent his signature as well against the eiruv. ( I am not stating the identity on the off chance said Rav has done Teshuva)