What classical composer was deaf when writing his 9th symphony?

Ludwig von Beethoven

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But of course, you're correct!

Around 1801, Beethoven began to lose his hearing. He suffered a severe form of tinnitus, a "roar" in his ears that made it hard for him to appreciate music; he would avoid conversation. The cause of Beethoven's deafness is unknown, but it has variously been attributed to syphilis, lead poisoning, typhus, and even his habit of immersing his head in cold water to stay awake. The oldest explanation, from the autopsy of the time, is that he had a "distended inner ear" which developed lesions over time.

Russell Martin has shown from analysis done on a sample of Beethoven's hair that there were alarmingly high levels of lead in Beethoven's system. High concentrations of lead can lead to bizarre and erratic behaviour, including rages. Another symptom of lead poisoning is deafness. In Beethoven's time, lead was used widely without an understanding of the damage it could lead to: for sweetening wine, in finishes on porcelain, and even in medicines. The investigation of this link was detailed in the book, Beethoven's Hair: An Extraordinary Historical Odyssey and a Scientific Mystery Solved. However, while the likelihood of lead poisoning is very high, the deafness associated with it seldom takes the form that Beethoven exhibited.

Over time, his hearing loss became acute: There is a well-attested story that, at the premiere of his Ninth Symphony, he had to be turned round to see the tumultuous applause of the audience; hearing nothing, he began to weep. - Wikipedia