Well, I did have one problem when I upgraded homebrew on Big Sur. For some reason I got the `brew link unbound` returns `/usr/local/sbin is not writable` error. I looked on Ask Different and found (for an earlier version of OSX) this [1] Basically, mkdir the directory.

[1] https://apple.stackexchange.com/questions/312330/brew-link-u...

that was the same with catalina when it first came out, same warning so I don't think this will be an issue indeed, also in catalina it worked fine regardless of the warning