返答ありがとうございます。
/app/Plugin にあった「CategoryExtensionB」を削除してから
プラグイン一覧に表示された「カテゴリページ/コンテンツ追加 for EC-CUBE4」の行の「インストール」ボタンを押してみました。
画面はインストール完了と出たのでうまくいったのかと思いましたが、プラグイン一覧には変わらず「インストール」ボタンが表示されています。
テーブル: dtb_pluginにもレコードが追加されていませんでした。
インストール画面に表示されたログは以下です。
引用:
<warning>Deprecation warning: require.ec-cube/CategoryExtensionB is invalid, it should not contain uppercase characters. Please use ec-cube/categoryextensionb instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require.ec-cube/UICube is invalid, it should not contain uppercase characters. Please use ec-cube/uicube instead. Make sure you fix this as Composer 2.0 will error.</warning>
<warning>Deprecation warning: require-dev.mikey179/vfsStream is invalid, it should not contain uppercase characters. Please use mikey179/vfsstream instead. Make sure you fix this as Composer 2.0 will error.</warning>
[183.3MiB/4.47s] Using version ^1.0 for ec-cube/CategoryExtensionB
[183.3MiB/4.47s] ./composer.json has been updated
[183.7MiB/4.61s] <warning>Deprecation warning: require.ec-cube/CategoryExtensionB is invalid, it should not contain uppercase characters. Please use ec-cube/categoryextensionb instead. Make sure you fix this as Composer 2.0 will error.</warning>
[183.7MiB/4.61s] <warning>Deprecation warning: require.ec-cube/UICube is invalid, it should not contain uppercase characters. Please use ec-cube/uicube instead. Make sure you fix this as Composer 2.0 will error.</warning>
[183.7MiB/4.61s] <warning>Deprecation warning: require-dev.mikey179/vfsStream is invalid, it should not contain uppercase characters. Please use mikey179/vfsstream instead. Make sure you fix this as Composer 2.0 will error.</warning>
[187.6MiB/4.82s] <warning>Dependency "ec-cube/plugin-installer" is also a root requirement, but is not explicitly whitelisted. Ignoring.</warning>
[187.4MiB/4.83s] Loading composer repositories with package information
[188.5MiB/5.87s] Updating dependencies (including require-dev)
[677.6MiB/30.84s] Nothing to install or update
[613.3MiB/30.92s] <warning>Package easycorp/easy-log-handler is abandoned, you should avoid using it. No replacement was suggested.</warning>
[613.3MiB/30.92s] <warning>Package facebook/webdriver is abandoned, you should avoid using it. Use php-webdriver/webdriver instead.</warning>
[613.3MiB/30.92s] <warning>Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.</warning>
[613.3MiB/30.92s] <warning>Package sensio/generator-bundle is abandoned, you should avoid using it. Use symfony/maker-bundle instead.</warning>
[613.3MiB/30.92s] <warning>Package setasign/fpdi-tcpdf is abandoned, you should avoid using it. No replacement was suggested.</warning>
[613.3MiB/30.92s] <warning>Package symfony/lts is abandoned, you should avoid using it. Use symfony/flex instead.</warning>
[613.3MiB/30.92s] <warning>Package zendframework/zend-code is abandoned, you should avoid using it. Use laminas/laminas-code instead.</warning>
[613.3MiB/30.92s] <warning>Package zendframework/zend-eventmanager is abandoned, you should avoid using it. Use laminas/laminas-eventmanager instead.</warning>
[614.0MiB/30.93s] Generating optimized autoload files
[615.1MiB/34.45s] Memory usage: 615.1MiB (peak: 683.26MiB), time: 34.45s
サーバー業者はプロセスを殺すような処理はないと言っていましたが、何が原因かよりわからなくなってきました。