Recommendation: ckeditor_details. It already ships in the Mantra codebase (2.2.0-beta2 in composer.lock) and is enabled on architect, classes and theme, but it is not on any text format toolbar yet. mantra_starter already styles .text-formatted details and the editor widget in libraries/global/component/details.css.

Other candidates from the drupal-code-query index, none a better fit: ckeditor5_bootstrap_accordion (439 installs, Bootstrap markup), ckeditor_bootstrap_tabs (1,923, tabs not collapsibles), ckeditor_readmore (738, a read-more toggle), ckeditor5_spoiler (52). ckeditor5_plugin_pack has no collapsible submodule.

ckeditor_details

3,487 installs, +1,213 over 12 months. Last commit Aug 2026, Drupal 12 work under way. Stable is 2.1.1; 2.2.0-beta2 (2026-10-13) requires ^11.4 || ^12.
Saves <details><summary>…</summary>…content…</details>. No front-end JS or CSS: the browser does the collapsing.
Native accessibility: keyboard support, expanded state announced. Chrome and Edge open a closed details element to show a Ctrl+F match. Works without JS.
One collapsible per block; no settings.
2.1.x wrapped the body in <div class="details-wrapper">; 2.2 drops the wrapper.

Round-trip test of the installed 2.2.0-beta2 plugin in a headless CKEditor 5 instance (setData → getData → setData → getData):

Insert from toolbar: <details><summary>Details</summary><p> </p></details>
Bold and links in the summary are kept
Headings and lists in the body are kept
Nested details are kept
2.1.x content with the details-wrapper div is unwrapped cleanly, so existing content upgrades without loss
<details open> loses open. The plugin only registers <details> and <summary>; not yet checked whether allowing <details open> in the text format keeps it.

Pasted AI content: <details>/<summary> HTML is upcast straight into the widget, so pasted collapsibles should come through as editable blocks (follows from the plugin's upcast converter, not tested with a real paste).

ckeditor_accordion

From reading the 2.3.0 source only; running the downloaded code was blocked, so nothing here was tested.

21,178 installs, +3,909 over 12 months. Last commit Jun 2025.
Saves <dl class="ckeditor-accordion"><dt>…</dt><dd>…</dd></dl>. Several rows per accordion, with add-row-above, add-row-below and remove-row buttons.
hook_page_attachments_alter() attaches 4 KB of JS and 3 KB of CSS to every page, admin pages included.
One site-wide settings page: collapse all, keep several rows open, animation, open a row from a #hash link, HTML in titles. The same for every accordion.
Weaker accessibility: titles are rewritten into <a href="#"> with no aria-expanded or aria-controls. Closed rows are display:none, so Ctrl+F can't find them. Without JS all rows show expanded.
Brings its own styling; it would need restyling for mantra_starter.

When ckeditor_accordion would still win

A true accordion: several rows managed as one block, one row open at a time, opening a row from a #hash link, animation. Native <details> can do one-open-at-a-time through a shared name attribute, but ckeditor_details has no way to set it.

Next steps

Pick the site and text format, add the Details button and allow <details> and <summary>
Check whether <details open> survives with the attribute allowed in the format