Extensions that are not compatible

As long as you run Catalina, you do nothing outside of inquiring of your apps' developers if they have made the changes needed.

Apple apparently have offered a workaround if you upgrade to Big Sur and have a needed app whose developer has been remiss in reacting to change notices Apple sent to developers over a year ago. See:

About system extensions and macOS - Apple Support

What extension is it and for what app?

Contact the developers for that app and see when they will be providing a version that is compatible with Big Sur.

If they don't plan on a compatible version then start looking for an alternative.