TYPO3 v12+, CKEditor 5: simple plugin that allows to <div> elements as wrappers around paragraphs. Not yet editor-friendly, though.

Custom.yaml

# CKEditor 5 configuration in TYPO3 (excerpt)
imports:
    - { resource: 'EXT:rte_ckeditor/Configuration/RTE/Processing.yaml' }
    - { resource: 'EXT:rte_ckeditor/Configuration/RTE/Editor/Base.yaml' }
    - { resource: 'EXT:rte_ckeditor/Configuration/RTE/Editor/Plugins.yaml' }

editor:
  config:
    importModules:
      - '@yourvendor/ckeditor-div'

    heading:
      options:
        - { model: 'paragraph', title: 'Paragraph' }
        - { model: 'heading1', view: 'h1', title: 'Heading 1' }
        - { model: 'heading2', view: 'h2', title: 'Heading 2' }
        - { model: 'heading3', view: 'h3', title: 'Heading 3' }
        - { model: 'heading4', view: 'h4', title: 'Heading 4' }
        - { model: 'formatted', view: 'pre', title: 'Pre-Formatted Text' }
        - { model: 'div', view: 'div', title: 'Div' }

    style:
      definitions:
        # block level styles
        - { name: 'Info alert (p)', element: 'p', classes: ['c-alert', 'c-alert--info'] }
        - { name: 'Info alert (div)', element: 'div', classes: ['c-alert', 'c-alert--info'] }
        - { name: 'Attention alert (p)', element: 'p', classes: ['c-alert', 'c-alert--attention'] }
        - { name: 'Attention alert (div)', element: 'div', classes: ['c-alert', 'c-alert--attention'] }
        - { name: 'Warning alert (p)', element: 'p', classes: ['c-alert', 'c-alert--warning'] }
        - { name: 'Warning alert (div)', element: 'div', classes: ['c-alert', 'c-alert--warning'] }

div.js

import {Core, UI} from "@typo3/ckeditor5-bundle.js";

// This plugin allows to use <div> elements as wrappers around paragraphs.
export default class DivElement extends Core.Plugin {
    static pluginName = 'DivElement';

    init() {
        const editor = this.editor;
        const model = editor.model;
        const view = editor.view;

        model.schema.extend('div', {allowIn: '$root'});
        model.schema.extend('paragraph', {allowIn: 'div'});

        editor.conversion.for('upcast').elementToElement({model: 'div', view: 'div'});
    }
}

JavaScriptModules.php

<?php

return [
    'dependencies' => ['backend'],
    'tags' => [
        'backend.form',
    ],
    'imports' => [
        '@yourvendor/ckeditor-div' => 'EXT:your_sitepackage/Resources/Public/JavaScript/CKEditor/div.js',
    ],
];

In order to add the selected elements to the div, I added this in model.change after const contentElement = writer.createElement('boxContent');:

// DocumentSelection
const selection = writer.model.document.selection;
// getSelectedBlocks() → IterableIterator<Element>
const blocks = selection.getSelectedBlocks();
if (blocks != 'undefined') {
    for ( const node of blocks ) {
        if (node != undefined && typeof node._clone === 'function') {
            const clone = node._clone(true);
            writer.append(clone, contentElement);
        }
      }
    }

and removed this:

-      // Beispielinhalt für das "boxContent" Element
-      const paragraph = writer.createElement('paragraph');
-      writer.append(writer.createText(''), paragraph);
-      writer.append(paragraph, contentElement);

Tested this with several selected p-elements.