In most of the pianos that we see today, the Sostenuto pedal does not exist. Only the pianissimo (una corda) and the sustain (damper) pedals can be seen. Throughout the time that I've been studying music, I have never come across any place to use the Sostenuto pedal of the piano. Whenever there is the need of a pedal to be used, most of the time it is the damper pedal. And when the piece should be played very soft (I mean softer than really soft), it is advisable to use the pianissimo pedal. But, I have no idea where to use the Sostenuto pedal on a piano. Of course its purpose is defined as: "sustaining a selected set of notes while keeping the others unaffected". Are there any examples of classical or modern music in which the Sostenuto pedal is used? Any links would be useful too.
You rarely need it, but when you do, it's indispensable! A few examples come to mind:
Claude Debussy: Clair de Lune. Second page, Tempo rubato. The left hand needs to hold some low octaves, then jump up to play some chords above middle C. Considering that it's Impressionistic music, and the dynamic marking is pp, you might be able to fudge it with the damper pedal, but it wouldn't sound as good.
Sergei Rachmaninoff: Prelude in c♯ minor. The sostenuto pedal is extensively used throughout the piece. You might be able to fudge the opening using just the damper pedal, but if you tried to play the last page using the damper pedal, it would be a horrible dissonant mess (especially with the sffff dynamic markings and accents!)
Maurice Ravel: Sonatine. The last two bars of the second movement require the sostenuto pedal. In fact, the left-hand chord starts with a tied grace note just before the beat, just to give you a chance to catch it with the sostenuto pedal.
Notice that the examples given above are all pieces written in the late 19th C and later. The sostenuto pedal wasn't even invented until 1844, so it makes sense that it would take some time before that was explored and used by many composers. Not only that, but just a little while later, piano began to be explored in different ways, particularly in regards to "preparations" and other ways of interacting directly with the strings, beginning in the 1920s. In my observations, since then, more people seem to have explored that avenue of writing rather than using the piano pedals to their fullest. Pianos have also had to compete with electronic instruments. In light of this, there is unfortunately not a whole lot of piano music that uses the sostenuto pedal.
As a composer, I am very aware of this capability on professional instruments and use it often. I have taken to naming the pedals Ped. I - sustain, Ped. II - sostenuto, and Ped. III - una corda. I do not think that I am unique in using these designations, but really cannot remember the first time I saw them (if I did). Here us an excerpt from my work Élégie, for euphonium and piano: