Twitch drops include two types of items: apparel items and consumable items.
Apparel items, such as T-shirts, are permanently saved to your account and will be available to you after each wipe (global or manual). After you create a new character, these items will be ready in the apparel section.
Consumable items, including weapons and gear, can be obtained only once and will not be sent to your account again if you wipe your character manually or after a global wipe.