Legacy systems?

I believe that the message is received when the particular app is opened. Concerning Apple’s comment regarding Legacy System Extensions, there is nothing we have to do or be worried about. What Apple is telling us is that when the next new operating system comes out (Big Sur), it will no longer allow those Legacy System Extensions which are part of other vendor’s apps. That means that any software vendors who have failed to update their offerings will be incompatible with the new OS. Those vendors have all been told what changes they will need to make to be compatible and I assume they are doing so. The bottom line is this. Before installing the next OS we need to visit our vendor web sites to verify that they have updated their offerings. If an important software has not been upgraded, then it might be a good idea to wait before installing Big Sur. At this time, it is expected that Big Sur is expected to be released late summer sometime. We will have to see how it all plays out. It shouldn't be a problem for most of us.

there are some info here How do you find legacy system extensions? - Apple Community