Using Drupal's Lenient Composer Endpoint
Drupal documentation for lenient composer endpoint.
Install modules that don't declare compatibility with your current Drupal...
#Composer
Resources
mglaman/composer-drupal-lenient
Updated Drupal Composer Lenient repository.
Asset Packagist
Download font end libraries using composer.
Instructions: https://www.drupal.org/docs/develop/using-composer/manage-dependencies
Composer Lock Diff
Use this tool to see what changed after running composer updates.