Is it safe to ignore the 'Packages' folder in Unity 2018?

manifest.json contains list of your dependencies(the packages you have used for your project). so if You clone your repository on a different machine, unity uses this list to download the required packages.

short answer: You should not put Packages in your .gitignore. It should be committed along with the rest of your project files.

Not a lot of google results for this, so for future searchers I will add:

It seems in Unity 2020.1 (and probably earlier) that the actual package contents are held in Library/PackageCache, and only the manifest file lives in the root Packages folder.

If Library is ignored as it should be, then there is no additional entry needed in the .gitignore.