The piano without its sostenuto pedal suffers from a loss of function
The sostenuto pedal, the middle pedal on a piano which sustains only specific notes - is regretfully an increasingly uncommon feature.
All upright pianos come either with two or three pedals. On a two-pedal piano, the left (soft) pedal decreases the volume slightly by bringing the rack of hammers closer towards the strings, while the right (sustain) pedal holds all notes played after the pedal has been depressed.
On three-pedal upright pianos, the middle pedal has been converted to a practice pedal. When depressed, an entire length of felt is moved over the piano strings, so that the hammers hit the strings through the felt. The result is a dampened sound, allowing one to practise longer and at unsociable hours at a much decreased volume. You can tell the look of a practice pedal straight away - if there is a reverse L-shaped notch in the middle for the pedal to be depressed and then locked by sliding to the left, it is a practice pedal.
Most digital pianos either come with one (sustain) or three pedals. In three-pedal digital pianos, volume can be electronically lowered, or headphones can be used to practise at lower volumes, so the sostenuto pedal has not been converted; on most grand pianos the middle pedal is also a sostenuto pedal. It sustains only the notes that are being held down as the pedal is depressed, while other notes remain unaffected. This function, introduced around 1844, makes it possible to have different layers of sound, some sustained, and some detached.
It is sad that sostenuto pedals are increasingly scarce, but it is a reflection that the bulk of composed piano repertoire does not utilise its function enough to warrant saving it. And if you are ever fortunate to play on a piano with one, relish the opportunity and experiment with the sounds!