Аlso and I can not understand why the abandoned Checkouts can not be deleted. Why then there is a Webhook - on Checkout deletion? And when triggers?
What I’ve found is that
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Checkout update Webhook contain the cart_token and you can delete the abandoned checkouts from your APP when Checkout update Webhook does not contain that token - eg. it happens only when abandoned checkouts are archived
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you can delete the cart from your APP right after Order Payment Success Webhook
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you can keep the Cart even if the User removes the items, once the user adds new items into Cart - only Cart Update Webhook is executed
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if the user clears the browser cache, and start adding items, Cart Create Webhook is executed, even that same Customer, hopefully, the APP create new Cart again. And of course, there can be one more abandoned checkout
So, to keep your APP updated and to not show to sellers x20 Carts full with items inside 
I will try to track abandoned checkouts based on cart_token