Undo History - Why limit it?
First off, this seemed like the best spot as Undo is a de facto user interface requirement for any editing program.
For school we use Photoshop CS3 and it seems it can only go back roughly 25 steps.
However, is there a valid User Experience reason why Adobe don't allow the ability to have infinite undo? Many other popular applications do offer infinite (or near infinite) undo such as:
Chrome (infinite <Back>)
Paint.NET (infinite undo)
Windows/File Explorer (infinite <Back> as far as I can tell)
MS Word (infinite undo)
The closest you can get to this in Photoshop is duplicating the layer you are going to modify and hide one of them.