Starting on February 3, 2020, Apple has adopted stricter notarization requirements for macOS Catalina applications. As a result, when installing Live2D Cubism Editor 4.0.04-beta1 or later in a macOS 10.15 Catalina or later environment, a warning popup containing the following message appears. Notarization is supported from 5.0.00 beta1, but for earlier versions, please try the following methods.

“Live2D_Cubism_Setup_(Version name)_en.pkg” can’t be opened because Apple cannot check it for malicious software.

When this occurs, you need to perform the steps below in order to launch the installer.

1. Use the Mac Finder and search for the downloaded installer “Live2D_Cubism_Setup_(Version name)_en.pkg.”

2. Right-click (Ctrl + Left-click) and select [Open] from the menu.

3. A dialog box appears. Click [Open]. When prompted, enter the administrator name and password.

For details, see the user guide issued by Apple.
https://support.apple.com/guide/mac-help/open-a-mac-app-from-an-unidentified-developer-mh40616/10.15/mac/10.15