True by design.
> and purely digital interfaces,
True but not relevant. Even if you connected an LCD monitor over an analog interface like VGA or composite video, it would have to re-digitize the signal. The fact that a screen uses a digital interface can be designed independently of whether a screen has fixed pixels (LCD, OLED) or not (monochrome CRT with continuous phosphor, without shadow mask or aperture grille).
> ensuring a fixed aspect (width-to-height) ratio
True by definition because the pixels are pieces of physical hardware in the screens.
> and completely square pixels.
This is a design choice. Nothing prevents someone from designing an LCD or OLED matrix with non-square pixels. But there's also no good reason to do so. (Side note: Due to RGB striping, single-color subpixels have an aspect ratio of 1:3, so those are non-square.)
Potentially useful trivia: Some image/video formats support declaring the pixel aspect ratio. Examples include PNG ( https://w3c.github.io/png/#11pHYs ) and Matroska.