Keeping the lens mounted on the camera, with caps on, does help reduce dust getting into the camera/lens connection area. For dust protection, that’s generally enough.

Fungus is different: it grows when gear is stored damp. The main goal is to keep the equipment dry and avoid condensation. Silica gel helps, especially if it is still active and periodically dried out/recharged.

A sealed plastic bag can be risky if the camera is bagged in warm air and then moved to a cooler place, because moisture inside the bag can condense on glass surfaces. That trapped moisture can encourage fungus.

Better storage options are:

• keep gear in a dry, well-ventilated place
• use fresh/recharged silica gel
• for long-term storage in humid climates, use a dry box or dehumidified cabinet
• occasional light/sun exposure can help, but dryness is the key factor

So: mounted lens = good for dust, but fungus prevention depends mostly on dry storage, not whether the lens stays attached.