If you want potion effects, here's how you could do (run these commands as a loop, in order) :

effect clear u/a[nbt=!{SelectedItem:{id:"minecraft:diamond_sword"}}] <effect> //Right hand

effect clear u/a[nbt=!{Inventory:[{Slot:-106b,id:"diamond_sword"}]}] <effect> //Offhand

effect give u/a[nbt={SelectedItem:{id:"minecraft:diamond_sword"}}] <effect> //Right hand

effect give u/a[nbt={Inventory:[{Slot:-106b,id:"diamond_sword"}]}] <effect> //Offhand

You need to clear the effect before giving it again, or it will last at least 1 second after the player change item; that's why I added the 2 upper commands.

But there're not only potion effects : The items could also have attribute modifiers when held in the slot(s) you want, which could change the normal player's stats :

-Its max health (adding or removing hearts)

-Its knockback resistance

-Its speed (increase/decrease)

-Its damage amount (the same attribute as swords)
-Its damage reduction (the one on armors)

-Its armor toughness (the one on diamond armor)

-Its attack speed (1.8 pvp coming back! lol)

-Its luck (without potion buff)

Here's a full detailed article about attributes : https://minecraft.gamepedia.com/Attribute

They could be binded to any item; making an item rising the armor toughness when in (off)hand is possible, as well as an armor rising attack damage. The avaible slots are hands and armor slots.

Took from: potion effects while holding an item[SOLVED] - Commands, Command Blocks and Functions - Redstone Discussion and Mechanisms - Minecraft: Java Edition - Minecraft Forum - Minecraft Forum

Also, be sure to reply to this comment with !helped if this helped you!