How find Legacy system extensions

There's no reason to worry about them at this time. Developers know which of their products will need updating to work with Big Sur and should/will offer updates before Big Sur is released to the public next year.

I second Old Toad's advice.

About legacy system extensions - Apple Support

If you still feel a need to see what they are, do "About this Mac..." from your Apple menu and, in the resulting window, click the "System Report" button. That opens the built-in utility, "System Information."

In that app's left-hand pane, scroll down the left pane to "Software" and, if needed, click the little arrowhead to expand the section. Find "Extensions" (see below) and select it.

Now you get to wait. The extensions info is a slow load. Have a good stretch. Once they display, click the "Obtained from" column heading to sort by the developer.

We can assume Apple will deal with theirs. That leaves a few from other developers, Some are used internally by Apple so we can expect them to be compliant in time for the changeover as well. A post in the the Logitech web site says their software will be compliant.

Even with this info in hand, you still need do nothing.