composer.json is not Drupal 9 compatible

8.x-1.5 released 16 March 2020 says

Requires Drupal: ^8 || ^9

but composer installation fails for me:

- Installation request for drupal/cookie_content_blocker ^1.5 -> satisfiable by drupal/cookie_content_blocker[1.x-dev, 1.5.0].
- Conclusion: don't install drupal/core 9.0.0
- drupal/cookie_content_blocker 1.x-dev requires drupal/core ~8.0 -> satisfiable by drupal/core[8.0.0, 8.0.0-beta10, 8.0.0-beta11, 8.0.0-beta12, 8.0.0-beta13, 8.0.0-beta14, 8.0.0-beta15, 8.0.0-beta16, 8.0.0-beta6, 8.0.0-beta7, 8.0.0-beta8, 8.0.0-beta9, 8.0.0-rc1, 8.0.0-rc2, 8.0.0-rc3, 8.0.0-rc4, 8.0.1, 8.0.2, 8.0.3, 8.0.4, 8.0.5, 8.0.6, 8.0.x-dev, 8.1.0, 8.1.0-beta1, 8.1.0-beta2, 8.1.0-rc1, 8.1.1, 8.1.10, 8.1.2, 8.1.3, 8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.1.x-dev, 8.2.0, 8.2.0-beta1, 8.2.0-beta2, 8.2.0-beta3, 8.2.0-rc1, 8.2.0-rc2, 8.2.1, 8.2.2, 8.2.3, 8.2.4, 8.2.5, 8.2.6, 8.2.7, 8.2.8, 8.2.x-dev, 8.3.0, 8.3.0-alpha1, 8.3.0-beta1, 8.3.0-rc1, 8.3.0-rc2, 8.3.1, 8.3.2, 8.3.3, 8.3.4, 8.3.5, 8.3.6, 8.3.7, 8.3.8, 8.3.9, 8.3.x-dev, 8.4.0, 8.4.0-alpha1, 8.4.0-beta1, 8.4.0-rc1, 8.4.0-rc2, 8.4.1, 8.4.2, 8.4.3, 8.4.4, 8.4.5, 8.4.6, 8.4.7, 8.4.8, 8.4.x-dev, 8.5.0, 8.5.0-alpha1, 8.5.0-beta1, 8.5.0-rc1, 8.5.1, 8.5.10, 8.5.11, 8.5.12, 8.5.13, 8.5.14, 8.5.15, 8.5.2, 8.5.3, 8.5.4, 8.5.5, 8.5.6, 8.5.7, 8.5.8, 8.5.9, 8.5.x-dev, 8.6.0, 8.6.0-alpha1, 8.6.0-beta1, 8.6.0-beta2, 8.6.0-rc1, 8.6.1, 8.6.10, 8.6.11, 8.6.12, 8.6.13, 8.6.14, 8.6.15, 8.6.16, 8.6.17, 8.6.18, 8.6.2, 8.6.3, 8.6.4, 8.6.5, 8.6.6, 8.6.7, 8.6.8, 8.6.9, 8.7.0, 8.7.0-alpha1, 8.7.0-alpha2, 8.7.0-beta1, 8.7.0-beta2, 8.7.0-rc1, 8.7.1, 8.7.10, 8.7.11, 8.7.12, 8.7.13, 8.7.14, 8.7.2, 8.7.3, 8.7.4, 8.7.5, 8.7.6, 8.7.7, 8.7.8, 8.7.9, 8.7.x-dev, 8.8.0, 8.8.0-alpha1, 8.8.0-beta1, 8.8.0-rc1, 8.8.1, 8.8.2, 8.8.3, 8.8.4, 8.8.5, 8.8.6, 8.8.7, 8.8.x-dev, 8.9.0, 8.9.0-beta1, 8.9.0-beta2, 8.9.0-beta3, 8.9.0-rc1, 8.9.x-dev].

I guess it's because a composer.json was added manually to the project which is incompatible. Is that possible?

See

  "require": {
    "drupal/core": "~8.0",
    "ext-json": "*",
    "php": "^7.1"
  }

Let's also see what the ProjectUpdateBotD9 says...