The following structures never generate naturally:

• minecraft:trial_chambers/corridor/addon/wall
  • has no jigsaw connector
  • adding a connector could cause the path to the entrance to be completely covered
  •  
• minecraft:trial_chambers/corridor/addon/reward_upper
  • has no jigsaw connector
• minecraft:trial_chambers/chamber/addon/middle_column_ranged_spawner
  • no structure references minecraft:two_column_start
•  minecraft:trial_chambers/chamber/addon/full_column_ranged_spawner
  • no structure references minecraft:three_column_start
•  minecraft:trial_chambers/chamber/addon/lower_walkway_platform
  • no structure references minecraft:lower_start
•  minecraft:trial_chambers/chamber/addon/walkway_extension
  • only referenced from the previous structure
•  minecraft:trial_chambers/chamber/addon/walkway_with_bridge_1
  • has no jigsaw connector
  • seems to be a development version of minecraft:trial_chambers/chamber/addon/c3_side_walkway_2
•  minecraft:trial_chambers/chamber/addon/middle_walkway
  • no structure references minecraft:lower_two_high
•  minecraft:trial_chambers/chamber/addon/c6_cover_small
  • has jigsaw block with no name
  • seems to be a development version of minecraft:trial_chambers/chamber/addon/c6_cover_small_1
• minecraft:trial_chambers/chamber/addon/c6_melee_spawner
  • has jigsaw block with no name

Furthermore, the following structures generate, but may be considered duplicates:

•  minecraft:trial_chambers/chamber/addon/short_platform and minecraft:trial_chambers/chamber/addon/short_grate_platform