TYPO3 InlineReadOnly FormDataProvider

ext_localconf.php

<?php
defined('TYPO3_MODE') or die();

call_user_func(function () {
    $GLOBALS['TYPO3_CONF_VARS']['SYS']['formEngine']['formDataGroup']['tcaDatabaseRecord'][
    \Vendor\Extension\Backend\Form\FormDataProvider\InlineReadOnly::class
    ] = [
        'depends' => [
            \TYPO3\CMS\Backend\Form\FormDataProvider\EvaluateDisplayConditions::class,
        ]
    ];
});

InlineReadOnly

<?php
namespace Vendor\Extension\Backend\Form\FormDataProvider;

use TYPO3\CMS\Backend\Form\FormDataProviderInterface;

/**
 * Disables all IRRE Controls and sets fields of IRRE records to readonly
 *
 * Note: Depending on the TYPO3 backend user permissions, a user may still be able to edit record content
 * (e.g. if table is available in record list and user has sufficient rights to edit data). But for inline integration,
 * the record(s) should be readOnly.
 */
class InlineReadOnly implements FormDataProviderInterface
{
    /**
     * @param array $result
     * @return array
     */
    public function addData(array $result)
    {
        $result = $this->evaluateInlineReadOnlyState($result);
        return $result;
    }

    /**
     * Evaluates 'readOnly' TCA state. Removes controls for IRRE elements with readOnly state and
     * sets fields of child TCA to readOnly
     *
     * @param array $result
     * @return array
     */
    protected function evaluateInlineReadOnlyState(array $result): array
    {
        // Disable all controls for the IRRE records
        foreach ($result['processedTca']['columns'] as $columnName => $columnConfiguration) {
            if (!isset($columnConfiguration['config']['readOnly'])) {
                continue;
            } elseif ((bool)$columnConfiguration['config']['readOnly'] === true) {
                $result['processedTca']['columns'][$columnName]['config']['appearance']['enabledControls'] = [
                    'info' => false,
                    'new' => false,
                    'dragdrop' => false,
                    'sort' => false,
                    'hide' => false,
                    'delete' => false,
                    'localize' => false,
                ];
                foreach ([
                             'inlineNewButtonStyle',
                             'inlineNewRelationButtonStyle',
                             'inlineOnlineMediaAddButtonStyle'
                         ] as $field) {
                    $result['processedTca']['columns'][$columnName]['config']['inline'][$field] = 'display: none;';
                }
            }
        }

        // Sets all fields to readOnly if parent inline element is readOnly
        if (isset($result['inlineParentConfig']) && $result['inlineParentConfig']['readOnly']) {
            foreach ($result['processedTca']['columns'] as $columnName => $columnConfiguration) {
                $result['processedTca']['columns'][$columnName]['config']['readOnly'] = true;
            }
        }

        return $result;
    }
}