Step 3 should probably be reinstalling the OS, and restoring data from backup (ideally from before the malicious version was installed).

And install ublock origin going forward.

Step one is to unplug the machine from the internet. Step two is to use another machine to change all your passwords, starting with the “pivot” passwords - your password manager master password, your email accounts, your AppleID, your mobile provider - followed by financial accounts and then all others. While changing passwords, make sure to “invalidate all sessions” where possible.

Only after you’ve done all this should you move onto Step 3: reformat your computer and install the OS from scratch.

Is there any way to check if you're affected? I just happened to install Homebrew while the malicious site was up and now I'm not sure if I installed the legit version.

Check if /tmp/update exists. If it does, you’re infected.

I think the malware tries to delete this file. So, it won't be a reliable method to identify whether you were infected.