The maturity date has nothing to do with whether the credit meets the definition of closed-end credit or open-end credit. Stop and think, all HELOCs have a maturity date, all credit cards have a maturity date (the card's expiration date).
Open-end credit is simply a credit product where the lender reasonably anticipates recurring transactions. See 226.2(20).
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The opinions expressed are mine and they are not to be taken as legal advice.