CKEditor 5 HTML Comments plugin supports commenting out individual list items, whereas the Drupal implementation strips out the commented-out item. This is with HTML correction plugins disabled on the text format.
Create text format
Assign CKEditor 5
Configure to use the source plugin
Configure to not use faulty HTML correction plugins
Create a list in the text field using CKEditor 5 text format
Source edit
Comment out a list item
Turn off source editing
Source edit
Note list item is removed
Leave the commented out list items in the HTML source.
Active
11.0 🔥
Last updated
With this markup, list item "beta" is removed when switching out of source mode.
<ul>
<li>
alpha
</li>
<!--<li>
beta
</li>-->
<li>
gamma
</li>
</ul>
I see this happening entirely in CKEditor without posting the form. I don't think Drupal's filters are involved.