yankel berel: I heard it from numerous Rabbonim. It was also printed on YWN in their article about Ponovezh in 2020: https://www.theyeshivaworld.com/news/israel-news/1854765/in-long-standing-minhag-israeli-flag-flies-from-ponevezeh-in-bnei-brak-on-yom-haatzmaut.html
R’ Ahron Soloveichik (who was a magid shiur in MTJ, then was appointed by R’ Hutner to give the highest shiur in Chaim Berlin, then at RIETS, before moving to Chicago where he was Rosh Yeshiva in Skokie before opening the Chicago branch of Brisk) also observed Tom Ha’atzmaut, and had his yeshivos say Hallel.