After the changed attribute modifiers format, the "show_in_tooltip" value is completely ignored.
In 1.20.6, this command will make the modifiers not show in the tooltip:
/give @p feather[attribute_modifiers={modifiers:[{name:"test",uuid:[I;0,0,0,0],type:"generic.gravity",amount:-2,operation:add_multiplied_base}],show_in_tooltip:0b}]
But in 1.21-pre1, the tooltip always shows the modifiers, even when "show_in_tooltip" is set to false
/give @p feather[attribute_modifiers={modifiers:[{id:"test",type:"generic.gravity",amount:-2,operation:add_multiplied_base}],show_in_tooltip:0b}]
Cannot reproduce. Tooltip does not show modifiers on the example item as given by the command in the description. Attached a screenshot.