Alphabet list not available from ckeditor ?

As far as I can tell, ckeditor only has numbered and bullet lists https://ckeditor.com/docs/ckeditor5/latest/features/lists/lists.html. Backdrop cannot alter that part. But you could install and try out TinyMCE editor and see if that has it.

That's cool that TinyMCE provides it out of the box!

Alphabetical lists are a type of ordered list, so if you want to keep using CKE5, you would have to set up some CSS styles and then provide the style to the dropdown "Styles" list button in the editor, in CKE5 "Styles" configuration something like (untested):

ol.alphabetical|Alphabetical list

And in your editor CSS file:

ol.alphabetical {
  type: A,
}