➜ ~ brew info test
Error: No available formula with the name \"test\"
==> Searching for a previously deleted formula...
Warning: homebrew/core is shallow clone
Under the cover Homebrew uses Git for version control, and a shallow clone is one that doesn't contain all history revisions/commits, for efficiency and data volume.
Actually, in most cases the warning can be safely ignored, as the formulae being searched for probably isn't available.
But if you're really looking for some formulae which might existed in the past, Just do what it suggests:
To get complete history run:
git -C "$(brew --repo homebrew/core)" fetch --unshallow
This way Homebrew could search for formula that existed only in the past but removed at some point.