Short answer: it depends on the Jain community and individual practice.

• Miso is a fermented soybean product. Many Jains avoid soy foods and fermentation, so miso is often considered not suitable for strict Jain diets.
• Jain views on food vary by tradition (Digambar vs. Shwetambar) and by person. Some may permit soy products if they come from a non‑harmful process, while others avoid them altogether.
• If you follow a strict Jain diet, the safest approach is to consult your own guru or community guidelines.