I think it would be useful to implement support for ".*ignore" files as an alternative to the ignore-dirs option. See for example what was done in prettier: prettier/prettier#2294 and prettier/prettier#2412

Pros:

• Most projects already have a .gitignore file and this could be automatically used. I don't think anyone wants do depcheck any paths ignored in .gitignore. I've seen depcheck crashing because it was not ignoring some of these files (in my case, huge binary files >= 100MB), it was difficult to troubleshoot.
• The syntax used in .gitignore and similar .ignore files is more powerful than the current ignore-dirs API. It allows using glob while the ignore-dirs API is only expecting exact matches on single level directory names. https://github.com/depcheck/depcheck/blob/master/src/check.js#L144

Cons:

• It's confusing to introduce duplicate functionality with ignore-dirs and an incompatible syntax. There needs to be a sensible transition strategy.