Damon Herriman did clarify that his Charles Manson doesn’t have a bunch of extra unseen footage. He seems to be specifically hanging on one other scene that he calls Quentin’s best (on the page anyway). Who knows? It may not have worked in the movie as much as Tarantino expected it to, so he made the call to cut it out. Or to satisfy the main stories revolving around Leonardo DiCaprio’s Rick Dalton, Brad Pitt’s Cliff Dalton and Margot Robbie’s Sharon Tate, he left Manson in the background.
Article source: https://www.cinemablend.com/news/2480236/once-upon-a-time-in-hollywoods-charles-manson-says-cut-scenes-were-quentins-writing-at-its-best