The hydraulic clutch slave shares the same brake fluid reservoir and should ideally be bled along with the brake fluid system, when its' bled. It's sorta like changing oil, but not changing the filter if the brake system is bled, but not the hydraulic clutch system. Fluid naturally gets old and ages even if the car sits and is never used. Symptoms of spongy clutch pedal, not being able to shift into gear are some indicators. Sometimes the focus is on cables and linkages when the culprit may be old accumulated air/water in the clutch line.