There might be a bit of confusion here. 1920x1080 is a 16:9 aspect ratio. If an image is a different aspect ratio, such as 3:2 or 4:3, then one should resize such images to 1080 pixels for the height, with 'contstrain proportions' ticked in Photoshop or other program.. The width will automatically be less than 1920.
If the image has a wider aspect ratio than 16:9, because it's a stitched panorama for example, then simply resize the width to 1920. The height will automatically be less than 1080 pixels.
If one can crop the image to 1920x1080 pixels, without detracting from its appeal, then that's the best option because you will get the most detail.
When presenting a slide show, one wants to present the maximum amount of detail. At least, I do. Cropping as close as possible to the 16:9 aspect ratio, without detracting from the essential message, should always be a consideration.