"This is a major upgrade. Please view the changelog for important details on this release and keep in mind that major upgrades may affect child theme customization. So use caution and update responsibly."
For those maintaining websites for clients, it's crucial to set up a maintainable system for keeping everything up-to-date and secure, but in a way where your client's live website never breaks.
WordPress makes it easy to update things with just a few clicks, but you still need to update responsibly. And when you're making what's deemed to be a "major upgrade" this is especially important. Keeping everything up-to-date regularly, backing up your website, and testing updates in a staging environment can help to avoid many problems.
Realistically, major upgrades may affect child theme customization, as well. This is another big reason to update responsibly and not push updates directly to your client's live site without testing.
Check out the following article about updating responsibly by Jason Bobich, the main developer behind Theme Blvd and Jump Start. It covers how to build a plan around updates, along with backing up and staging your changes before pushing them live.
View Article: Updating Your WordPress Website Responsibly