Will historical data lead to target leakage?
I'm going to kind of ignore your question. To me it sounds like you have a dataset containing essentially employee starting and 'end dates' or if the employee is still working for you no end date. This sounds like the standard case for a class of regression techniques called 'survival regression' or survival analysis. I suggest you look into this.

Now for your question, Im not too sure about target leakage as I cannot quite see how, if you have a structural change in employees year by year then theres definitely some leakage yeah.