Legacy Extension?? what should I do

Maybe, but in any case, you should remove TotalAV from your system using the developers instructions.

According to: About legacy system extensions - Apple Support

"Some system extensions will not be compatible with a future version of macOS."

I think Apple did a very poor job of explaining this. But hopefully they handle it better once it happens.

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.