Dropbox sent me a message to allow something. Now apple says legacy system extensions will not work with macOS newer versions. What does this mean?

Hello ej157,

Welcome to Apple Support Communities!

The message you're seeing is letting you know that some extensions may not be compatible with the current macOS running on your Mac. The article you came from, About system extensions and macOS, provides the information about the message you've received.

"System extensions work in the background to extend the functionality of your Mac. Some apps install kernel extensions, or kexts—a kind of system extension that works using older methods that aren't as secure or reliable as modern alternatives. Your Mac identifies these as legacy system extensions.

In 2019, Apple informed developers that macOS Catalina will be the last macOS to fully support legacy system extensions, and we've been working with developers to transition their software.

If you get an alert about a system extension

You may see an alert on your Mac that says that a program loaded or tried to load a system extension signed by the developer of that extension. 

• The alert may ask you to open Security & Privacy preferences to allow the extension. You may also need to restart your Mac. 
• The alert may ask you to contact the developer for support because the extension needs to be updated or will be incompatible with a future version of macOS.
• The alert may inform you that it will damage your computer and has been blocked.

On a Mac with Apple silicon, you may first need to use Startup Security Utility to set the security policy to Reduced Security and select the “Allow user management of kernel extensions from identified developers” checkbox.

System administrators can also use Mobile Device Management (MDM) to manage legacy system extensions."

Let us know if you have any questions.

Thank you for using Apple Support Communities.

Take care!