Could any more app info be provided such as which apps using legacy system extensions?

Even if you knew what products were causing this, the offending software resides on the read-only Catalina Systems partition and you cannot remove these offending items causing this dialog. This is one of those dialogs without any actionable purpose.

One can only hope that an upgrade to the released Big Sur will purge these sources of pointless dialogs.

If you have an insatiable appetite to see what these legacy items might be, you can press the option key and select System Information… and once that panel appears, select Software -> Legacy Software. Another approach can be taken in the Terminal:

kextstat -a | grep -v com.apple | awk 'NR>1 {printf "%s\t%s\t%s\n", $6,$7,$8}' | more



Hold down the Option key, go to the  menu and select System Information. Then go own the sidebar to "Legacy Software" in the Software section:

Actually there's little for you to do or worry about. When Big Sur is released the developers will or should have offered updates or upgrades to their software if they want to keep customers who move on to the new system.

Some developers, like Print Shop did with Catalina, will opt to not support their product beyond the current version.