When a steel dragon is about to die from old age, it supposedly goes into hiding to recall all the memories of all the human lives it has lived, but what if a steel dragon is killed during battle while still in its human form?
Will it, just like any other creature using polymorph, revert back to its original form?
Will the dragon just teleport away from battle if it's almost on death's door? But what happens when the dragon isn't able to escape?
It reverts to its draconic form.
In AD&D 2e, D&D 3.5, and D&D 4th edition, a steel dragon reverts to its true dragon form when slain.
In AD&D 2nd edition, according to the Monstrous Compendium: Greyhawk Adventures Appendix, the steel dragon's shapechanging ability comes from the ability to use polymorph self five times per day with unlimited duration. According to the AD&D 2e (revised) Player's Handbook, p. 206, polymorph self ends when the caster is slain.
In D&D 3.5's stats for the steel dragon on the Wizards of the Coast website (also Dragon #339), the steel dragon's shapechanging relies on the spell-like ability to polymorph five times per day. This spell also ends when the creature is slain (D&D 3.5 Player's Handbook, p.263).
In D&D 4e, the steel dragon's ability to assume human form (Draconomicon: Metallic Dragons, p.172) references the Change Shape ability (Monster Manual 2, p.216), which specifically ends when the creature dies.