Franz Schubert died one year after Beethoven. He wrote nine symphonies, but the Eighth (the "Unfinished") is only two movements long. It is not known whether Schubert intended to complete it. The Ninth (which was rehearsed but never performed while Schubert was alive) was eventually brought forward by Robert Schumann, and Mendelssohn conducted the premiere. In any case, the Schubert Ninth was originally published as his Seventh. He also started working on a Tenth, but it is no more than a few piano sketches.
Bruckner completed eight symphonies and three movements of his Ninth. He came close to breaking the "curse" but died before he could finish the finale of his last symphony. The fact that he spent much of his time revising and editing his earlier symphonies might have had something to do with his inabiltiy to finish the Ninth. However, it should be noted that Bruckner's second completed symphony, which was later shelved by the notoriously insecure composer, has survived as the Symphony No. 0 or "Die Nullte."
Gustav Mahler was a superstitious man, and he was determined to beat the "curse." After finishing his Eighth Symphony (the choral "Symphony of a Thousand"), Mahler started working on a new five-movement symphonic song cycle. However, instead of numbering it, he published this "ninth" symphony as The Song of the Earth. He then completed his Ninth Symphony (really his Tenth) and started working on the Tenth. He sketched out most of the melodies, defined the structure, orchestrated the first movement of the five-movement work, and then died of heart failure in 1911 at age 50. Most conductors play only the finished first movements. Some, like Sir Simon Rattle, elect to play the completed orchestration by noted musicologist Deryck Cooke.
By the way, Dmitri Shostakovich "officially" broke the curse in the 1950s with his Tenth Symphony. The Russian composer went on to write fifteen.