I'm writing a novel in third person limited point of view. I want my prologue to feature an event that is referenced later in the novel, but my protagonist is not involved in the event. (e.x the prologue is a murder, the novel has the protagonist searching for the killer). If my novel is written with the protagonist as the narrative focus, is it all right for my prologue to have an objective pov? Thank you!
As the author of the story, with absolute freedom to tell your story anyway you want, you can use any POV you want in any scene you want. So yes, you can use an Omniscient POV for the prologue and 3rd person limited in the scenes of your chapters.
I am told that 3rd Person limited is the most common POV used for writing these days because it is the easiest to do well. Whereas omniscient POVs are less common, but this POV is hard to use well. Tolkien and Frank Herbert wrote in Omniscient POV and were master storytellers.
My understanding is the challenge in using Omniscient POV, especially 3rd person or Subjective Omniscient POV, is that technically the narrator knows everything that will or has happened -- they can be anywhere in time and space and, because of that 3rd person or Subjective modifier, in any character's head. So the narrator knows how the story ends and what is going to go wrong or right for the protagonists and the antagonists. That means that if the narrator doesn't share something really important about the story with the reader, something that is critical to understanding the what and why of everything in the story about to be told, that it's easy for the reader to feel like they are getting jerked around. But, sharing all that information works against creating suspense and tension.
Since you are limiting your Omniscient POV to your prologue, it may not be too much of a risk. You can mitigate it by restricting yourself to a straight Omniscient POV, one without any insight into a character's thoughts, and only relating character's dialog and actions.
But, then, no one reads Prologues anyway, so it may not really matter at all what you disclose.
R. A. Freeman invented the "Inverted" detective story, in which the reader is first shown the crime in detail, and then sees how the detective solves it. In most cases the first section is told from the criminal's PoV, and the 2nd from the detective's or an associate of the detectives (the Dr Watson role). But in some cases, the first section was done in an Omniscient Pov. This worked well IMO.
I have seen an Omniscient prologue specifically used in works of fantasy, that then followed one or more characters in tight-third.
Like any other technique, this is fine when well done, not so much when poorly done.