How do I know what legacy system extensions are affected on my MAC?

There is nothing you need to do about this. It is app developers' responsibility to make sure their products keep up with computer makers' changes. You can't make the changes. Apple is simply giving you a heads-up about future changes.

To me, those advisories simply add evidence to consider as you decide whether a future system software update is appropriate for you.

See: About legacy system extensions - Apple Support

You don’t need to worry about this until macOS 11 “Big Sur” is released later this year and there is no law that says you have to update to it either. As for legacy extensions you will receive a pop-up notification when one is encountered. Reputable developers will have already updated their products to comply with Big Sur’s requirements. If this still concerns you might wait awhile before installing Big Sur and keep a lookout for problems others might encounter.

I second the advice given here. I came across a site that apparently lists the status of various apps, which you can check. Apparently it changes regularly. It has a current list of compliant/ non-compliant apps with extensions:

https://forums.macrumors.com/threads/big-sur-working-not-working-apps.2242312/

See this thread: https://discussions.apple.com/thread/251242953?answerId=252384802022#252384802022

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