Regardless of values in the slots field, when launching a melee attack, value effects of minecraft:damage are always active if the enchanted item is in mainhand, and are always inactive if the item is in other slots.
The same thing happens to value effects of minecraft:armor_effectiveness.

Steps to reproduce

1. Install the attached datapack mc272006.zip
2. Enchant a sword with the command: /enchant @s test:increase_damage_in_offhand
3. Hit some mob with the sword in mainhand
4. Hit some mob with the sword in offhand

Expected behavior
Mobs are one-hit killed when the sword is in offhand.

Observed behavior
Mobs are one-hit killed when the item is in mainhand.
The enchanted_hit particles are also produced when hitting mobs with the item in mainhand.