When you add new dependencies to your elixir project and run mix deps.get, it will create/update the mix.lock file, that list all information about the versions of the dependencies that you are using, such as version, source, etc.

But when you remove a specific dependency from your mix.exs file, you will need to remove it from the mix.lock file to keep the file as updated as possible.

But, NEVER update this file manually, let mix do it for you:

mix deps.unlock NAME

This command supports some arguments:

--all - unlocks all dependencies
--filter - unlocks only deps matching the given name
--unused - unlocks only unused dependencies (no longer mentioned in the mix.exs file)
--check-unused - checks that the mix.lock file has no unused dependencies. This is useful in pre-commit hooks and CI scripts if you want to reject contributions with extra dependencies

So to keep your mix.lock file updated, just run:

mix deps.unlock --unused --check-unused

Important note: This command only unlocks dependencies, to remove it completely, including build artifacts and fetched sources, you can use the following command:

mix deps.clean --unused --unlock