Turns out there’s some kind of build issue with htop; the Homebrew formula has been modified to not install on High Sierra.

Here’s what I got when attempting to upgrade; for the record, htop still works fine for me:

    ==> Upgrading 1 outdated package, with result:
    htop 2.0.2_1
    htop: This formula either does not compile or function as expected on macOS versions newer than Sierra due to an upstream incompatibility.
    Error: An unsatisfied requirement failed this build.