The phrase "which was sought" is correct and usable in written English.
You can use this phrase to indicate that something was requested or sought after. Here is an example sentence using the phrase: "The evidence was destroyed, which was sought by the opposing party."
Her release, which was sought by the United States, came two days before Secretary of State Colin L. Powell was to arrive in Beijing.
For when once the moisture has been set in motion it is not easily brought to rest, until the idea which was sought for has again presented itself, and thus the movement has found a straight course.
That which was sought to be accomplished was simply to improve navigation on the waters in question so as to meet the wants of the vast commerce passing and to pass over them.
The inquiry, requested by China, Norway and Switzerland, will be made by the same panel of trade experts that will investigate the first complaint, which was sought by the European Union, Japan and South Korea.
The overall bill, which was sought by the National Collegiate Athletic Association and many prominent coaches and athletes, was approved by the committee on a voice vote and sent to the floor.
But it looks likely that Ethan's flight to Mexico will have the effect of bringing about the separation from his mother, at least, which was sought at his sentencing, although not in the way the court anticipated.
So I joined Inclusive Church, which was seeking a paid worker.
Another factor was the Kremlin, which was seeking new displays of geopolitical muscle.
Amherst also defeated the 1942 Williams team, which was seeking a perfect record.
He even joined the ruling party, which was seeking to gain Igbo support.
He enrolled at the training academy for the New York City Police Department, which was seeking Arabic speakers.
