This is Tarantino's tribute to what he saw as a golden time in Hollywood. Tate's murder symbolized a loss of innocence that changed the industry forever.
However, the title acknowledges that this is all a fairy tale. The parallel plotlines of Rick Dalton and Cliff Booth suggest that the industry and the culture were already changing, and that change is inevitable. Dalton, once a leading man in his own TV series, is struggling against changing tastes and the next generation of actors. Old-school Booth battles the hippies who have taken over his old stomping grounds, but there is the feeling that his kind are on the way out, and the hippy revolution is just around the corner.
I think Manson's presence is really open to interpretation. One reading is that he's simply a red herring. I think another way of reading his survival underscores the admission that this is a fairytale fantasy, and that in real life, very bad people exist and get away with horrible things.
Tarantino's first three films are almost entirely character-focused, dialogue-driven thrillers that rely entirely on the charisma and depth of their characters. Reservoir Dogs could not function without the tense interplay between Mr. White, Orange, Blonde, and Pink. Pulp Fiction is made great by the (at the time) completely unique relationships and dialogue between its characters, especially Jules / Vincent and Butch / Marcellus. Jackie Brown is probably Tarantino's most human and compassionate film, and it is certainly his least cartoonish and most grounded film; it is proof that Tarantino can make a great film even when he cuts back his quirky stylistic choices.
Once Upon a Time in Hollywood felt like a call-back to Jackie Brown in many ways which was just so refreshing for me. I have missed the Tarantino who is so interested and invested in not just his characters but the relationships between the characters. I've missed the Tarantino who understands restraint is just as important as stylized action or quirky dialogue.
