There does exist an edition which ends with her killing herself because Sir Stephen leaves her.Originally Posted by Curtis
While the edition is of questionable authenticity (and as you say, Reage implicitly disowned it by bringing O back for a sequel), the plot line isn't totally improbable from what I know of intense D/s relationships.
In the first place, the end of the relationship is hinted at in a line near the end, where O catches SS's eye at an unguarded moment and realises for the first time that he loves her. I know some old school Masters who got rid of a slave they fell in love with, on the basis that they couldn't be totally in control of her if they loved her.
And in the second place, I have myself had to counsel abandoned slaves - some of them strong, independent women just like O - who felt they had nothing left to live for without their Master. It happens.