There have been indeed some changes to Elixir’s Logger since v1.15.0:

[Logger] The :console configuration has been deprecated in favor of :default_formatter
[Logger] The :backends configuration has been deprecated in favor of Logger.add_handlers/1

Logger now uses :logger’s default handler and refers to the logger_std_h documentation, which states:

type = io:standard_io/0 | io:standard_error/0 | file | {device, io:device/0 } - Specifies the log destination.

The value is set when the handler is added, and it cannot be changed in runtime.

Defaults to standard_io, unless parameter file is given, in which case it defaults to file.

So once the require Logger is excuted in the one-liner, the default handler will have been added and its device cannot be changed during runtime. And looking at the actual
configuration of the default handler:

elixir -e 'require Logger; :logger.get_handler_config(:default) |> IO.inspect(); Logger.error("why stdout?")' 2>scr/delme
{:ok,
 %{
   id: :default,
   module: :logger_std_h,
   config: %{
     type: :standard_io,
     burst_limit_enable: true,
     drop_mode_qlen: 200,
     flush_qlen: 1000,
     sync_mode_qlen: 10,
     overload_kill_restart_after: 5000,
     burst_limit_max_count: 500,
     burst_limit_window_time: 1000,
     overload_kill_enable: false,
     overload_kill_mem_size: 3000000,
     overload_kill_qlen: 20000,
     filesync_repeat_interval: :no_repeat
   },
   level: :all,
   filters: [remote_gl: {&:logger_filters.remote_gl/2, :stop}],
   filter_default: :log,
   formatter: {Logger.Formatter,
    %Logger.Formatter{
      template: ["\n", :time, " ", :metadata, "[", :level, "] ", :message, "\n"],
      truncate: 8096,
      metadata: [],
      colors: %{
        error: :red,
        enabled: true,
        info: :normal,
        warning: :yellow,
        debug: :cyan,
        notice: :normal,
        alert: :red,
        critical: :red,
        emergency: :red
      },
      utc_log?: false
    }}
 }}

08:53:01.187 [error] why stdout?

it is indeed using stdio.

It is possible. You just need to add additional Logger backend that will filter messages to level :error or higher and output to stderr.

I took the freedom to edit some code fences in.

As this:

some text [ ]

renders into

some text

Where the is a clickable checkbox.