Damage is considered to be an effect of an attack. However, Holon WP specifies "effects of attacks, except for damage, done to the Defending Pokémon". Anything that involves damage calculation is unaffected by Holon WP. Special conditions, placing damage counters, "smokescreen" status, removing energy from the Defending Pokémon, etc. are all prevented by Holon WP.
Even something that doesn't directly specify the Defending Pokémon may also be prevented. For example,
EX: LM Roselia's Flick Poison reads:
"Switch 1 of your opponent's Benched Pokémon with 1 your opponent's Defending Pokémon. Your opponent chooses the Defending Pokémon to switch. The new Defending Pokémon is now Poisoned."
Even though the effect of the attack is focused on the Benched Pokémon, the Defending Pokémon is removed from the Active slot from the attack. Therefore, Flick Poison does nothing against a monster with Holon WP.
For something like
δ Crobat's Radar Jam and
EX: UF Stantler's Push Away, the attack is a universal effect, and is unaffected by Holon WP.
This may sound somewhat confusing, but I hope you can at least understand what I am trying to say.