how to remove legacy system extension

You don't. If you do so with current operating systems, you computer could stop working. If only affects upcoming OS versions.

You cannot fix anything; software developers have to deal with their own products. Please see this Apple article:

About legacy system extensions - Apple Support

If you are referring to removing 3rd-party KEXT files (kernel extensions) you can use a free tool like KnockKnock from https://objective-see.com.

I had two 3rd-party audio interface extensions for that wouldn't work properly in Catalina and couldn't for the life of me get them removed and was able to do it easily with this tool. It also helped me find a couple more that I had entirely forgotten about.

I think Apple did a very poor job of explaining this.

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.