I'm a newbie in SwiftUI development, but I had the same question for my macOS application.
The answer I've found seems a little bit ugly, but using SwiftUI-Introspect library you can just remove corresponding toolbar item. Maybe it will be helpful for someone.
.introspect(.window, on: .macOS(.v10_15, .v11, .v12, .v13, .v14)) { nsWindow in
if let toolbar = nsWindow.toolbar {
if let index = toolbar.items.firstIndex(where: { $0.itemIdentifier.rawValue == "com.apple.SwiftUI.navigationSplitView.toggleSidebar" }) {
toolbar.removeItem(at: index)
}
}
}