How do i delete legacy system extension

The next major release of macOS will not allow third party kernel extensions. It's just warning that any third party software you're using that installs such extensions will not work in macOS 11.

All vendors who now user kernel extensions will have to use a different method to load items at startup.

https://support.apple.com/en-us/HT210999

The message should have mentioned the name of the vendor the extension is from. But in short, all third party extensions are in the following location, along with ten default extensions installed with the OS.

/Library/Extensions

For now, you don't need to do anything.

Yes, but any third party app you have that installs and uses a kernel extension will stop working in macOS 11. All third party vendors will have to update their apps to use the new system Apple has put in place for Big Sur.

As an example, MalwareBytes has already noted they are working on it.