Joomla! Documentation™

Theoretically, the pre-update check would tell you if your third-party extensions are compatible with J5. However, the pre-update check is only helpful if all extension developers have made their extension reflect compatibility with their extensions. In a perfect world, the Extensions portion of the pre-update check would tell you if an extension either:

• Can be upgraded without the backward compatibility plugin enabled
• Can be upgraded with the backward compatibility plugin enabled
• If an update to the extension is required before upgrading from J4 to J5
• If an extension is incompatible completely