Root folders disappeared after update to Big Sur

Under Mojave and earlier version, I have created a few folders under the root folder of Macintosh HD. I know that this is not where it's designed to be, but still I was shocked when I discovered that upgrading to Big Sur (macOS 11) would remove all of my folders without even the slightest notice.

Has my data been saved to some other folder or is it lost?

Solution 1:

The data's not been deleted.

It's been moved to /System/Volumes/Data and can be retrieved from there.

It's recommended to move the data to your own user directory, or to /Users/Shared, if it's meant to be accessible by all users of this Mac.