Q.pepper plants withered leaves, yet, no spots.
Most of my indoor pepper plants, by the large window, their leaves have or are withering and dying. They do not have SPOTS. Their leaves were all healthy and green, then one by one their leaves began to mostly wither away. Bernie.

Certified GKH Gardening Expert
It could be a wilt. This article will explain more:
https://www.gardeningknowhow.com/edible/vegetables/pepper/pepper-wilt-on-plants.htm
It may also be a lack of water. Potting soil can actually start to repel water if it gets too dry. You might try flooding the container to make sure the soil is fully wet.