How do I uninstall the legacy system extension?

That warning you get is not telling you to do anything. It is the developers' responsibility to make sure they have a new version that does not use extensions. Ignore it, or ask the makers of your "must have" apps.

About legacy system extensions - Apple Support

From an Applications -> Utilities -> Terminal session you can issue the following command to see what 3rd party kernel extensions your system is currently using:

kextstat | grep -v com.apple

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 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.

One additional comment. If you were to identify the legacy extensions and then remove them, the apps they are associated with would no longer function, So don't go there.