If Earth were flat, there would never be a reason why you can't see the sun from an airplane at cruising altitude as there would be nothing to block sunlight
If Earth were flat you'd be about to see the same constellation of stars any where in the planet.
If the Earth was flat, and the sun didn't set, but "goes away," then Earth would have to be far larger than what was is told to us. However, that would mean that travel times would be much longer than actually are, which clearly isn't the case when traveling from one place to another.
If Earth was flat, then the sun and Moon would have to change size, becoming smaller as they "set" beyond the horizon. Actually, the sun and Moon appear larger at the horizon, when juxtaposed to objects on earth buildings, trees, ships, etc.
If Earth were flat, summers in the southern hemisphere would experience less day light than the its winter months.
If Earth were flat, it would be impossible for Australia and Chile to experience daylight during the summer while Alaska is experiencing 24 days without daylight. Sunlight would have to reach Alaska for that to be possible, and Chile and Australia are on opposite sites of the planet.
If Earth were flat there would be no lunar eclipse.
If Earth were the would remain completely round as it left your view.
Everything I've mentioned can be seen for yourself without any "conspiracies" from whoever.