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I posted a question on the Google Help Forum last night, and this morning I got a reply, which I have paraphrased for your amusement.

Table of Contents for a spreadsheet can be seen by using the All Sheets button.
It is located on the bottom row on the left side.
It is the 'hamburger' icon (3 horizontal lines).
Clicking on it shows you a list of the individual sheets that are in this spreadsheet document.