I have 16 Dark American Arborvitae (Thuja occidentalis Nigra) planted in 2009. They were at least 3 years old at that time, as they were about 6" toll.
During last two years they lose a lot of green at the lower part of each tree. Is that the normal life cycle, or something else, please?

Can you post some pictures of the whole plants and closeup of the bottom of a representative plant?

Are the bottoms in the shade?

One side of the bottom has some shade from the fence in the north side of the trees. The south side is 100% sun.
I will take a picture and bring it to the Forum soon.

Did they go brown and drop or did they disappear?

These are the pictures of the Thua. #91 is the view from South. You can see white walls of the neighbor house through the tree. Two yours ago we could not see these walls. That means a lot of green leaves are gone.
#92 is the side view of the North side of the trees. The fence is about 4.5" high.

They look really healthy. I think the bottom branches are being shaded out by the fences and the tops of the arborvitae.

Normal

Agreed. Normal and old growth shaded out.

Thank you. Although I would prefer them to get more leaves and block the view!

Some Arborvitae can act like Scotch Pine with a lot of open space.

These are Dark American Arborvitae (Thuja occidentalis Nigra).