Why did Stephen pretend to have a limp in Django Unchained?

Whilst Steven may not have 'lost' his limp, there is definitely a suggestion that he is deliberately exaggerating his physical handicap...

The entire film is a portrait of ethnic stereotypes, with each character seemingly tackling and subverting hackneyed clichés; In Stephen's case, 'The Uncle Tom'...

The phrase "Uncle Tom" has also become an epithet for a person who is slavish and excessively subservient to perceived authority figures, particularly a black person who behaves in a subservient manner to white people; or any person perceived to be a participant in the oppression of their own group

'Uncle Tom' characters are widely portrayed as bumbling, dithering and slightly comically inept court jesters, with their incompetence often emphasized by some trivial malady... Which in Stephen's case is absolutely on the nose.

The Uncle Tom or 'House Nigger' is such an established trope, Malcolm X drew attention to it directly in one of his speeches:

"Back during slavery," Malcolm begins, "there were two kinds of slaves. There was the house Negro and the field Negro. The house Negroes -- they lived in the house with master, they dressed pretty good, they ate good 'cause they ate his food -- what he left. They lived in the attic or the basement, but still they lived near the master; and they loved their master more than the master loved himself. They would give their life to save the master's house quicker than the master would … Whenever the master said 'we,' he said 'we.' That's how you can tell a house Negro."

In the case of Stephen, this subservience is merely a ruse, as we later learn when Stephen discusses his discovery of Schultz and Django's ruse in the drawing room...

His voice, his manner of address (particularly to Candie), his diction and his subservience all adapt or evaporate instantly. Could you imagine the 'Stephen' from earlier within the film helping himself to brandy, and sitting in the masters chair?

By outward appearances, Stephen is a largely impotent character: but secretly he is cold, calculating and incredibly observant. The limp is part of his guile to fool those around him into dismissing him, and this malice aforethought is what Jackson used to try and create "The most hated Negro in cinematic history."

I don't think Stephen pretended to limp. Right from the his first appearance till his last he had been limping, or using a stick to walk. In the climax, when he faces Django he knows that he is about to die. His throwing of stick and saying that he counted 6 bullets was an act of defiance, as if he was telling Django, that he was not afraid of him, and not afraid of dying. Please note that we do not see Stephen walking much after he throws his stick. He just gets down two steps after throwing stick. It seems that he must have had some kind of walking problem and that's why he used stick. So not pretending at all.