Question

What are all the things Defog removes?

Answer 1

Generation IV
Defog lowers the target's evasion one stage, even if the target is behind a substitute. It bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of a move such as Dig or Fly.
Defog clears away any fog on the field, as well as Light Screen, Reflect, Safeguard, Mist, Spikes, Toxic Spikes, and Stealth Rock on the target's side of the field.

Generation V
Defog can now be reflected with Magic Coat and no longer lowers the evasion of a Pokémon behind a substitute.

Generations VI and VII
Defog now clears away Spikes, Toxic Spikes, Stealth Rock, and Sticky Web on both the user's and target's side of the field as well as Aurora Veil on the target's side of the field. It still only clears away Light Screen, Reflect, Safeguard, and Mist from the target's side of the field.
If powered up by a Flyinium Z into Z-Defog, the user's accuracy is raised by one stage.

Generation VIII
Defog now also clears terrains and the effects of G-Max Steelsurge.

source

Comments

The first ones to answer get left behind :3

Answer 2

Defog lowers the target's Evasion by one stage and removes entry hazards and barriers on both sides of the field. The moves affected are: Light Screen, Reflect, Mist, Safeguard, Spikes, Stealth Rock, Sticky Web and Toxic Spikes.

Hope I helped :)

Comments

Also terrain

Answer 3

Effects
Defog lowers the target's Evasion by one stage and removes entry hazards and barriers on both sides of the field. The moves affected are: Light Screen, Reflect, Aurora Veil, Mist, Safeguard, Electric Terrain, Grassy Terrain, Psychic Terrain, Misty Terrain, Spikes, Stealth Rock, Sticky Web, G-Max Steelsurge, and Toxic Spikes.

Defog also clears foggy weather in battle (which causes Accuracy loss when present), however this weather condition is only present in Sinnoh (Diamond/Pearl/Platinum) and cannot be created in battle otherwise.

Source: https://pokemondb.net/move/defog

Hope this helped :)