The Year Over Year Comparison can be found in TurboTax Online by going to "Tax Home" on the left-hand side menu and selecting Review then Year Over Year Comparison. The year-over-year comparison only shows the current tax year and the previous tax year.
Also be aware that the year-over-year comparison is only available if you transferred your previous year's return into your current year's return.

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That's why we need the .tt23 file when going from the online version to the download version. Turbotax doesn't make it possible.