As of Minecraft 1.20.5, hiding attributes with item flags no longer works for vanilla attribute modifiers.
This is caused by Mojang managing attribute modifiers 2 different ways (default modifiers are not part of the item/nbt itself)

Here are a few workarounds:

Hide Components:

The simplest solution is hiding a component.
Minecraft added a Tooltip Display component in 1.21.5.
This allows you to hide any component from the item you don't want to be seen.
You can use SkBee's Tooltip Style Component.
Example:

set {_i} to 1 of diamond sword
apply tooltip display component to {_i}:
	hidden_components: attribute_modifiers

give player 1 of {_i}

Remove Vanilla Attribute Modifiers

A simple solution is just removing the vanilla attribute modifiers from the item.

[!IMPORTANT] Keep in mind this will remove all vanilla modifiers.
If you want to keep vanilla modifiers, see Copy/Hide Vanilla Modifiers

set {_i} to diamond sword with nbt from "{""minecraft:attribute_modifiers"":{modifiers:[],show_in_tooltip:0}}"

Apply/Hide Empty Modifier

Another solution is adding a useless modifier then hiding it.

set {_i} to diamond boots

# Apply a useless modifier to the item (since its amount is 0, it wont do anything)
apply attribute modifier to {_i}:
	id: "my_mod:scale"
	attribute: scale
	amount: 0.0
	slot: feet_slot_group
	operation: add_number
# Now hide in tooltip
add hide attributes to item flags of {_i}
give player {_i}

Copy/Hide Vanilla Modifiers

This method will copy the vanilla attribute modifiers into the item and then hide them

set {_i} to diamond boots

# Add all the vanilla modifiers to the custom modifiers of the item
loop all attribute types:
	add default loop-value modifier of {_i} to loop-value modifier of {_i}

# Then hide them
add hide attributes to item flags of {_i}
give player {_i}