Craft CMS issue with Composer V2
Composer 2 support has been drafted over at craftcms/plugin-installer#4. We’ll merge that in once Composer 2 is officially released.

~~In the meantime, if you really need to get Craft working with Composer 2, follow the instructions at craftcms/plugin-installer#3.~~

UPDATE: craftcms/plugin-installer and yiisoft/yii2-composer have both been updated to support Composer 2.