Legacy extensions. What do I need to do?

At the moment you don't need to do anything. Certain system extensions will no longer function in the next release of macOS, 10.16. There will be more information available closer to the end of the year.

However, the vendors for the apps you are currently using that depend on a kernel extension, will need to modify their app and provide an update. Whether they choose to do that at all, or to do it for free, or to charge a fee is something you would need to discuss with the vendors.

From an Applications -> Utilities -> Terminal session you can issue the following command to see what 3rd party kernel extensions your system is currently using:

kextstat | grep -v com.apple