How do I determine which software uses Legacy system extensions?

Hold down the Option key and go to the  menu and select System Information. In the sidebar under Hardware looks for Legacy Software.

That will give you a list of such software. FWIW I checked and have none. Got rid of them when I updated to 64 bit apps for Catalina. There's really nothing you need to do as the developers will be updating their software to work with Big Sur and following systems.

I think Apple did a very poor job of explaining this. But hopefully they handle it better once it happens.

You should always wait a few iterations of a new OS to get the bugs worked out, that's likely a year from now!

No need to worry, but to see them, just don't upgrade to 10.16 or iOSXi, or whatever Big Sur is if the developers don't have updates...

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

Hmmmm, I have plenty of legacy SW & that shows none!???