In Pathfinder we had the Squeezing rule
In some cases, you may have to squeeze into or through an area that isn’t as wide as the space you take up. You can squeeze through or into a space that is at least half as wide as your normal space. Each move into or through a narrow space counts as if it were 2 squares, and while squeezed in a narrow space, you take a –4 penalty on attack rolls and a –4 penalty to AC.
When a Large creature (which normally takes up 4 squares) squeezes into a space that’s 1 square wide, the creature’s miniature figure occupies 2 squares, centered on the line between the 2 squares. For a bigger creature, center the creature likewise in the area it squeezes into.
A creature can squeeze past a creature while moving but it can’t end its movement in an occupied square.
To squeeze through or into a space less than half your space’s width, you must use the Escape Artist skill. You can’t attack while using Escape Artist to squeeze through or into a narrow space, you take a –4 penalty to AC, and you lose any Dexterity bonus to AC.
In 2ed could't find anything similiar. Did I miss something?
That action also specifies that it's for exceptionally tight spaces, and that a lot of small spaces are just difficult terrain. I would be pretty hesitant to use that to declare that moving through any space smaller than your usual square allotment is impossible in combat.
In the lack of a formal rule, though, I think the practice largely comes down to a judgement call, based on the body type of the creature, whether a space would be difficult terrain, or impassable without great time and flexibility.