Export - Image Sizing. Add a check box for (Lock Aspect Ratio).

I agree that to stretch an image to suit an output, would not be within the general spirit of LrClassic. But to crop an image suitably on the fly for output would be. And that would permit outputting the same image to multiple aspect ratios without needing to generate virtual copies and crop those, or alternatively, without turning to the Print module which already can do this, but which carries some undesirable consequences.

Perhaps a toggle-switch.

When resizing is ON, choices for this switch would be "fit within" / "crop to shape". With the former, all current behaviour would continue. With the latter, an on-the-fly crop override could happen which reduces the image (centred) according to whatever aspect ratio is implicit in the requested width and height. This option should therefore become unavailable whenever "Shorter Edge" / "Longer Edge" / "Megapixels" sizing was selected.

Or perhaps as a refinement, when this was active, one could choose either to state Width AND Height, or else to state Longer AND Shorter edge.

When resizing is OFF, this toggle should instead choose between "entire crop" (current behaviour, all  sizing options except Resolution greyed out) and "crop extract". The latter new mode would allow defining a width and height extract boundary in pixels, which the image would be cropped to directly on the fly (centred in current crop), and this would be what was exported. If such an extract crop ever strayed outside the image boundary, the overflow area should still be included - as white pixels.

Ease of outputting a certain-sized 1:1 extract has been often requested here. As has, the ability to in effect maintain different aspect ratio versions of the same image but without needing to repeat all processing across multiple image versions.

I feel that something like the above suggestions could satisfy many use-cases efficiently, and without offending any good-practice principles.