Conversation so far:
What instruments were added to symphony orchestras?
Other instruments such as the piano, harp and celesta may sometimes appear in an orchestra.
What was the point of Beethoven adding instruments to symphonic orchestras?
Beethoven's use of piccolo, contrabassoon, trombones, and untuned percussion—plus chorus and vocal soloists—in his finale, are his earliest suggestion that the timbral boundaries of symphony might be expanded.

Question: Did this become common practice?
Self-contained rewrite: Did adding instruments to symphonic orchestras become common practice?