Answer:
Under the Regulation DD advertising requirements, the only place you can use the APY abbreviation without having "annual percentage yield" spelled out at least once is an indoor sign. On an outdoor sign such as a digital display board a CD ad that includes the APY must also include the minimum balance to earn the displayed APY, the term of the account, and, if applicable (rare), the disclosure required by ยง1030.8(c)(6)(iii).
A digital display board that is located inside the building is not an outdoor sign, even if it's only visible from outside the branch. See the "Editor's Note" on this page: https://www.bankersonline.com/regulations/12-1030-008