The following is a part of an article about the linguistic diversity of New York City by Ross Perlin:
The great migrations, increasingly diverse till early 20th century, came to a sudden halt with the immigration act of 1924, with its hard cap on immigrants and its racist quotas in favor of Northern and Western Europe. The city's diversity was becoming just a little less radical. By the time the United Nations arrived in New York City in the 1950's, it was a mostly a town of seven particular tribes: Irish, Italian, Jewish, African American, Puerto Rican, West Indian, and Dominican. This is what now many think of as the dynamic "old New York" -- but it was the most static the city has been, linguistically speaking, since its founding.
I don't know how to read the sentence indicated by bold type. I guess that it has the meaning something like "At that time the city was linguistically in the most static period in its entire history". But, I don't understand how it is constructed grammatically.
EDIT 17 OCT
Thanks to the responses to my question, I understand the meaning of the sentence now. But, I still don't understand how it is constructed.
(1) it was the most static the city has been since its founding.
A sentence containing a relative clause usually can be divided into two sentences:
(2) Thursday was appointed as the day the trial would be held.
(3a) Thursday was appointed as the day.
(3b) The trial would be held at the day.
But it seems sentence (1) cannot be divided in this way.
And, a relative clause usually restricts the interpretation of the antecedent. But, in (1) the antecedent (the most static) seems to restrict the meaning of the relative clause (from the entire history of the city to a particular period in it). I found a sentence which has a similarity to (1) .
(4) He looked me up the last time he was in Tokyo.
I think we can add extra words to make it more usual form (though awkward):
(5) He looked me up the last time of the occasions when he was in Tokyo.
Can we add some words to (1) to make it usual form? Is this kind of analysis irrelevant?