PluginManager.php

<?php

    public function loadRepository(RepositoryInterface $repo)
    {
        foreach ($repo->getPackages() as $package) { /** @var PackageInterface $package */
            if ($package instanceof AliasPackage) {
                continue;
            }

            switch ($package->getType()) {
                case 'composer-plugin':
                    $requiresComposer = null;
                    foreach ($package->getRequires() as $link) { /** @var Link $link */
                        if ($link->getTarget() == 'composer-plugin-api') {
                            $requiresComposer = $link->getConstraint();
                            break;
                        }
                    }

                    if (!$requiresComposer) {
                        throw new \RuntimeException("Plugin ".$package->getName()." is missing a require statement for a version of the composer-plugin-api package.");
                    }

                    // if the required Plugin API version is exactly "1.0.0", convert it to "^1.0", to keep BC
                    if ($requiresComposer->matches(new VersionConstraint('==', $this->versionParser->normalize('1.0.0')))) {
                        $requiresComposer = $this->versionParser->parseConstraints('^1.0');
                    }

                    $currComposerPluginApiConstraint = new VersionConstraint('==', $this->versionParser->normalize(PluginInterface::PLUGIN_API_VERSION));

                    if (!$requiresComposer->matches($currComposerPluginApiConstraint)) {
                        $this->io->writeError('<warning>The "'.$package->getName().'" plugin was skipped because it requires a Plugin API version ("'.$requiresComposer->getPrettyString().'") that does not match your Composer installation ("'.PluginInterface::PLUGIN_API_VERSION.'"). You may need to run composer update with the "--no-plugins" option.</warning>');
                        continue 2;
                    }

                    $this->registerPackage($package);
                    break;
                case 'composer-installer': // Backward compatibility
                    $this->registerPackage($package);
            }
        }
    }

Note that incompatible (version mismatch) plugins are not registered anymore. (line 33)