Poor text on a non-Retina display
Q
My MacBook Air mid-2013 runs Big Sur well, but its display renders text blurred and lacking contrast, worse than on an older model. Why is that, and what can I do?
A
As your MacBook Air doesn’t have a Retina display, the death knell will have been macOS Mojave, which removed subpixel anti-aliasing or rendering, used in older versions to improve the appearance and readability of screen fonts. While you can try changing the Font smoothing setting in the System Preferences > General pane, that only adds linear blend antialiasing, which isn’t the same.
There are some ways you could try to improve this in Terminal, but you should also consider whether the age of your display is partly to blame. Backlit LED screens do deteriorate with age, and after nine years it’s unlikely to have as crisp and high contrast an image as it had originally. If you want to see more life from your Mac, consider wiping it and installing High Sierra, which should restore some of its former glory, although it’s just as unsupported now as Mojave.