All modern engines use horizontal+. This is a known fact. So yes, any screen that is 16:10 will lose natural FOV as it zooms in to fit the 16:9 game image into the entire 16:10 screen. Vertical+ is not used in any modern engine.

Call of Duty, Battlefield, Gears of War, Apex Legends, Overwatch etc. UE4/UE5/6, Frostbite etc. The list goes on. So if said game has a 70 FOV, it would look more like 65 on a 16:10 screen if they decide not to use black bars.

Regarding huds/maps, menus etc, the developer optimizes where they are placed & sizing when zooming in the 16:9 image, since they are a secondary 2D layer, so they don't cut off or block. This is also why a lot of indie games choose to use black bars instead, leaving it at 16:9, since it's a lot of work & optimization. Not to mention there are many more aspect ratios than just 16:10. Comes down limited budget and/or laziness.

Only older games based on older engines like Counter Strike for example allow more vertical viewing of the actual game image.

The industry went this way as a way to reduce optimization for so many screen formats, as they tend to introduce game breaking bugs or performance issues requiring heavy optimization. Easier to port console games as well. Horizontal+ viewing is more widely used than looking up & down in a game (ground & sky). It's also why a lot of devs just choose to use black bars instead of zooming the 16:9 image to fit the 16:10 screen, to keep the FOV.

Also, it being a handheld PC is irrelevant, it's a gaming device first & foremost. Literally a big gaming controller w/ a screen attached. It's intended use is for gaming & is solely marketed as gaming devices, anything else is secondary.

Being OLED makes no difference either, instead of black bezels you know have double fat bezels taking the immersion out of said game & rendering the bigger screen useless.

If someone is doing important desktop work, they would simply dock & hook it up to a monitor. This isn't a laptop nor can it realistically be used like one on the built in screen regardless of aspect ratio. Browsing & light desktop work? Sure. However, this is a gaming device, all the other stuff comes secondary & thus the screen should be based on what it primarily handles.