Authentication required (git.drupalcode.org) On Drupal 9, I want to install the Taxonomy menu module using Composer, but I get the following error.

Using version ^3.4 for drupal/taxonomy_menu
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 0 updates, 0 removals

Installing drupal/taxonomy_menu (dev-3.x e89d1e8):
Authentication required (git.drupalcode.org): Username:


The command is waiting for a Git username/password, but two days ago the installation ran smoothly.
How can I ask for a stable version of the module?
On the project's page they write that, as of 21 May 2019, Taxonomy menu 8.x-3.x-dev is already fully compatible with Drupal 9.
This must be why composer require initially proposed 3.4 version and ultimately tried to install the dev-3.x version.
How can I ask for a stable version of the module?

You can't install a stable version of Taxonomy Menu module for Drupal 9 at the moment because no such version exists.
If you look at the project page, you see the latest version is 3.4 which is marked Requires Drupal: 8.x.



So, if you want to use a stable version, you have to stay with Drupal 8 for now.
To use the dev version on Drupal 9, you can do this in composer.json:
    &quot;drupal/taxonomy_menu&quot;: &quot;3.x-dev&quot;,

Normally you should not need to authenticate to git.  It's probably a proxy issue; you might find some hints in this discussion.