U2720Q, missing Auto Resize aspect ratio option

@Ubermacsimus  Does the U2720Q's on-screen interface confirm that it's actually receiving a signal at a resolution lower than its native resolution?  The reason I'm asking is that on newer PCs or Windows versions (not sure which), if you choose a lower resolution than the display's native resolution, Windows will render at that lower resolution, but will keep the "active signal resolution" at the display's native resolution.  There are some benefits to doing this (switching resolutions doesn't cause a black screen, and it allows a lot more flexibility when dealing with a handful of displays that don't have built-in scaler chips), but if the signal actually on the wire matches the display's native resolution, then it's never going to offer scaling options because it won't see a need to scale.  In that case, you'd change scaling options in the Control Panel application of your GPU, i.e. Intel HD Graphics Control Panel (or Command Center depending on driver version) or NVIDIA Control Panel.

If on the other hand the on-screen menu confirms that it's currently receiving a lower resolution, then I'm not sure what to tell you.  I do remember a similar case where somebody saw an option documented in the manual of a display (USB-C Prioritization) but couldn't find it on their display, and it turned out that the option in question was added starting with a newer hardware revision of that display than the person had, and it wasn't mentioned anywhere that older examples wouldn't have the option.

Personally I don't understand why the specific Aspect Ratio settings described in the manual would even be offered.  If you want 4:3 or 5:4, then I would expect the user would just send that type of resolution, not send a 16:9 resolution and squeeze it to one of those less wide resolutions.  I could understand having options like Fit (fill as much display area as possible while maintaining aspect ratio and avoiding cropping), Fill (fill entire display area even if it means stretching), Zoom (fill entire display area while maintaining aspect ratio by cropping edges), and 1:1 pixel mapping, but that doesn't appear to be what's offered.

Yes, the monitor says I'm sending a 1024x768 @ 75Hz signal, which I am, so I think your second explanation would be more likely, that the Auto Resize option may be available in a future firmware update.

@Ubermacsimus  Just purely out of curiosity, why would you send a 1024x768 signal to a 4K display?  If you scale while maintaining aspect ratio, you're going to have a lot of physical display real estate unused due to the aspect ratio difference, and the 1024x768 pixels from the signal won't scale cleanly onto the 2880x2160 physical pixel grid that would be used to display that image, and 1024x768 gives you a minuscule amount of workspace by today's standards.

I'm using the U2720Q in a KVM setup that includes some vintage computers that can't go beyond 1280x1024. I'd like those to display 4:3 automatically, ideally, when I switch to them. I also have modern machines connected that run 4K on the monitor just fine.

I used to run a pair of old Dell U2410 screens that could switch aspect ratios automatically like this. It's annoying that the U2720Q can't do the same.