1619 led to interesting discussions. If I understand correctly, there is a book in progress to clarify/correct some subjects in the NYT articles.

Regarding 1776, I remember Frederick Douglass'  speech "What to the Slave is the Fourth of July"? It is read on the holiday by many schoolchildren. It was a call to activism.

https://www.npr.org/2017/07/05/535624532/what-to-the-slave-is-the-fourth-of-july-frederick-douglass-revisited

The authors of the complaint letter never bothered to contact the 1619 Project authors as noted by Atlantic journalist Adam Serwer.

In the age of social-media invective, a strongly worded letter might not seem particularly significant. But given the stature of the historians involved, the letter is a serious challenge to the credibility of the 1619 Project, which has drawn its share not just of admirers but also critics.

Nevertheless, some historians who declined to sign the letter wondered whether the letter was intended less to resolve factual disputes than to discredit laymen who had challenged an interpretation of American national identity that is cherished by liberals and conservatives alike.

“I think had any of the scholars who signed the letter contacted me or contacted the Times with concerns [before sending the letter], we would've taken those concerns very seriously,” Hannah-Jones said. “And instead there was kind of a campaign to kind of get people to sign on to a letter that was attempting really to discredit the entire project without having had a conversation.”

https://www.theatlantic.com/ideas/archive/2019/12/historians-clash-1619-project/604093/

Edit to add:

The letter will lead to closer scrutiny of the planned book series.

https://moguldom.com/241418/book-series-planned-based-on-new-york-times-1619-project-new-way-of-thinking-about-american-history/