The Rebbe's Letter Concerning Publicizing the Completion of a City Eruv, 1 Adar 5718
The Rebbe advised Rabbi Yosef Dovid Moskowitz not to publicize the completion of a city eruv. His concern was the lasting public habit created by the announcement.
An eruv can later become invalid
A wire, post, wall, gate, or connection can fail after the route has been accepted. Weather, construction, utility work, or a new opening can change the enclosure between one Shabbos and the next.
The inspectors and Rabbonim may learn of the break, but notice will not necessarily reach every person who heard the original announcement.
Publicity creates a lasting habit
Once carrying becomes ordinary, families arrange their Shabbos around it. Strollers, keys, food, and seforim move through the streets without anyone revisiting the halachic basis each week.
The effect of a later failure is therefore not confined to the builder or inspector. It reaches every person who continues acting on the public heter.
A valid eruv without public announcement
In a separate typed response, the Rebbe describes a case in which a discreet eruv can be worthwhile: the place has no Torah-level reshus harabim and is otherwise suitable under halacha.
The two ideas fit together. A valid eruv can provide protection, while avoiding publicity prevents a broad habit of carrying that may continue after the boundary fails.