Should delta.yaml be committed to git? Getting started with CraftCMS 3.1's project config files and I can't find any documentation about whether storage/config-deltas/delta.yaml should be committed to git
No, you would not check these files into Git.

I have a .gitignore rule to just ignore everything in storage/ and so I treat it as a cache directory.

Which it is... except for the unfortunate fact that any rebrand (logo, icon) files are stored in storage/ as well. But I just manually add them in via git add -f