One thing that still bothers me with this module is that it never was (and isn't right now) compatible with HTML code coming from elsewhere which just contains . The only appears properly if it was inserted before with the button. If, for example you switch to the source code view and enter somewhere, it won't appear when you switch back to the editing view and so on. And if you insert an nbsp; with the button and switch to the source code view you'll see the <nbsp> auxiliary tags.
With CKE4 there probably wasn't a great solution for this. With CKE5 this seems to have changed:
https://github.com/ckeditor/ckeditor5/issues/10898
If I understand correctly, it might very well be possible now to upcast just the so that it appears as an element in the editing view but not in the source code editor and not when you save. I think this might prove to be a much more elegant solution in the future. So I'd just wanted to mention this for the record. It might be a good idea to investigate this path in the future.
PS: This might also make the filter unnecessary, BTW
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