Homebrew installs iftop into /usr/local/sbin and not in /usr/local/bin so it’s not a part of your $PATH by default.

Yes, you can add it manually via .zprofile or something similar, but there is a native way to add new paths to all shells of all users:

echo '/usr/local/sbin' | tee -a /etc/paths

Or you can add a new entry to paths.d:

echo '/usr/local/sbin' | tee -a /etc/paths.d/homebrew