I only have 1.13 + OTP 25 right at hand to test, but on that version I still get the same warning from plug when running mix compile --warnings-as-errors with an empty _build directory.

However, --warnings-as-errors doesn’t turn warnings in dependencies into errors.

Some historical digging reveals that it used to; around the time of Elixir 1.4 that was changed, by commits like:

Just use --warnings-as-errors which is a sane option for CI

For example:

==> plug
Compiling 1 file (.erl)
Compiling 41 files (.ex)
warning: System.stacktrace/0 is deprecated. Use __STACKTRACE__ instead
  lib/plug/conn/wrapper_error.ex:26: Plug.Conn.WrapperError.reraise/3


==> ecto_sqlite3
Compiling 4 files (.ex)
warning: function query_many/4 required by behaviour Ecto.Adapters.SQL.Connection is not implemented (in module Ecto.Adapters.SQLite3.Connection)
  lib/ecto/adapters/sqlite3/connection.ex:1: Ecto.Adapters.SQLite3.Connection (module)

Elixir’s code states that it is a known issue and they just don’t want to add breaking change

        # TODO: Consider raising instead of warning on v2.0 as it usually cascades

I’m curious about what these are, because I can’t remember the last project I worked on that didn’t build with --warnings-as-errors in CI.