$ brew update
... . . .
Warning: You are using macOS 10.12.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Discourse, Twitter or IRC. You are responsible for resolving any issues you
experience while you are running this old version.

「古いバージョンで動かしてる限り何かあってもおめーの自己責任だからな」
きっつー