You have a drawing object Anchored to page in this undeletable page. For some unknown reason, this text box containing “CC BY-SA 3.0 and a URL” is invisible and not selectionable. It is permanently anchored to page 6 and whatever you do this page 6 will exist for ever, even if you reduce your document to its first page.
Anchoring frames or objects to page is a common error. Although pages do not exist per se in Writer, anchor mode To page creates a static permanent page with this number. Text may flow as usual through it but the page and its position is now an immutable object.
I found no way to make this text box editable again. There is still the possibility to manually patch the XML but I don’t recommend it if you can do otherwise because you could damage irremediably the document (or at least the copy). However, I did it to check and your page 6 disappeared as expected.
Also, using a text box is usually not what you want. A text box is meaningful in a form but if you’re writing an “ordinary” text document, the right “object” is a text frame which creates a secondary text flow with all the usual formatting properties, which a text box does not possess.
Also, why did you change the hierarchy of Heading 1 to 6 (and Title as well) to make them dependent on your user style normal instead of the usual Heading? I also noticed you have 118 character styles ListLabel n which is a clear indication you missed something in the general style mechanism.
Since your text boxes can’t be deleted, restart your document from scratch if it does not contain a lot of text yet. Otherwise, copy you text in a fresh document. I emphasized word text to insist upon the fact you must copy paragraph per paragraph to make sure you don’t copy the drawing objects by accident.
Avoid using any drawing objects unless you really need graphics in it (a text box is just another form of graphic object). Even in this case, it is better to draw your graphics in Draw and copy/paste the result as a single image. Text frames are your best friends for side insertion of short text like margin notes or “floating” captions.
Never anchor To page until you perfectly know the non-conventional properties of this mode and its consequences.
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