Why is composer self-update command missing?

[int-ua]

$ ./bin/composer self-update --2
  Command "self-update" is not defined.

https://drupal.stackexchange.com/q/299757/7177

[int-ua]

And composer 2.x is impossible to install

$ ./bin/composer prohibits composer/composer '^2.0'
drupalcommerce/project-base  dev-master  requires          composer/composer (^1.0)
drupal/core-dev              8.9.13      requires          composer/composer (^1.9.1)
composer/composer            2.0.0       requires          composer/semver (^3.0)
drupalcommerce/project-base  dev-master  does not require  composer/semver (but 1.7.2 is installed)