How can I identify which programs are using these legacy extentions?

Concerning Apple’s announcement 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. In the meantime, there is nothing we need to do.

There may be a way that I don't know of, but you could visit the web sites of some vendors you are concerned about and see if they say they are, or will be, Big Sur compliant. This kind of information is generally always provided.

About this Mac>System Report>Software>Extensions, click on Obtained from Header, all that aren't Apple are Legacy.