As per the information provided on https://www.minecraft.net/de-de/article/minecraft-snapshot-24w10a, the slot name player.cursor is not intended to function in creative mode, particularly within the regular inventory. However, this restriction doesn't extend to GUIs other than the regular inventory. When examining a GUI other than the standard inventory, the slot name does indeed work in creative mode. To replicate this, input the command

item replace entity @a player.cursor with air

into a repeating command block. You'll observe that items picked up with your cursor in creative mode are deleted when viewing a chest, crafting table, etc.