There's currently no webhook specifically for when a checkout becomes abandoned, an abandoned checkout is just a checkout like you'd receive in a checkouts/create webhook that never ends up becoming an order.
Because of that, you could possibly work something out by subscribing to both checkouts/create and order/create webhooks combined. You'd have to find a way to store the info from each webhook and compare the two though - and if an orders/create webhook is received that containts a checkout_id of a checkout you've gotten a checkouts/create webhook from, that checkout could be considered no longer abandoned.
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