Automating additions to the Spotlight ignore list used to be as simple as using defaults write, but the release of macOS 10.15 Catalina brought some changes that make it slightly more complicated:

• The VolumeConfiguration.plist file has moved. After upgrading to Catalina, you now have multiple volumes even though Finder shows the volumes combined. The config’s actual location is now: /System/Volumes/Data/.Spotlight-V100/VolumeConfiguration.plist.
• defaults no longer seems to work on VolumeConfiguration.plist. Instead, it now complains the domain does not exist.

PlistBuddy does still work, although it’s less elegant. Thankfully, there are some straightforward examples in the man pages that we can copy:

sudo /usr/libexec/PlistBuddy -c "Add :Exclusions: string /path/to/folder/" /System/Volumes/Data/.Spotlight-V100/VolumeConfiguration.plist

Once you’re done editing the ignore list, a restart of your computer should trigger Spotlight to detect the changes.¹ You may also be able to trigger it by opening the Spotlight Privacy preferences, or just waiting around for the next index.

But… why?

In practice, I’m using this to automatically ignore dependency folders like node_modules in my dotfiles. Quite often, one package I’m working on depends on another, and it’s annoying for Spotlight and Alfred to display a packaged version instead of my development copy. Automating them away on all my machines makes life just a little bit easier.