"almost all" is greatly exaggerating. Mode X was much less used than EGA or VGA 320x200 modes. As a consequence, many games look completely wrong with a standard DOSBox configuration these days (characters are fat!), because it still doesn't use aspect ratio correction by default.
It's not exaggerating; I cannot remember a single VGA title made after '91 that uses plain mode 0x13. Primary reason being not being able to double buffer. By that time, EGA titles were also practically dead, the notable exception being Civlization.
Sure, there were games pre-91, but in the '90, the PC market exploded, together with available games. The amount what was available in '90s dwarfs the older titles, so "almost all" is greatly fitting.
It could also double-buffer, unlike the standard mode 0x13.