Are HSA subject to the requirements of Reg DD?

If they are held in a deposit account - yes:

Sec. 230.2 Definitions.

For purposes of this part, the following definitions apply:

(a) Account means a deposit account at a depository institution that is held by or offered to a consumer. It includes time, demand, savings, and negotiable order of withdrawal accounts. For purposes of the advertising requirements in Sec. 230.8 of this part, the term also includes an account at a depository institution that is held by or on behalf of a deposit broker, if any interest in the account is held by or offered to a consumer.