Martin requested the code be left there so that should anyone else want to make use of it they could.
No reason other than that for the code to be there.
To be truthful I see no reason not to remove it from master now,
MDL-22732 was a long time ago and I don't believe there have been any requests to have it put back.
Of course our normal procedure from deprecating + removing a function should be followed here just in case, as Michael points out it still has the potential to cause regressions, it being a protected method would still allow someone to override the class and make use of it.
My +1 to add a debugging notice informing any developers this function will be removed in 2.5, integrating that in this issue, and opening an issue targeting 2.5 to remove that function entirely.