The first time I used AE, we had a power cut. The next time I opened the application I went to check my preferences and the option to set a limit on 'undo's has completely disappeared from the general preferences panel. Is the application damaged/corrupted?
I tried uninstalling and reinstalling AE but the 'undo's limit is still missing...
The preference for controlling the number of undo levels was removed in After Effects CC (12.1). The number of undo levels is 99, the maximum.
I presume that the update was installed between the last time you changed this value and when you noticed the change.
More information about what's new in After Effects CC (12.1) is here:
Information about what's new in After Effects CC (12.2) is here:
As Tim says, we removed that preference. The document that he points to gives the explanation, but there is the relevant excerpt for convenience:
"default number of Undo operations available increased from 32 to 99, and preference removed from user interface
We have increased the default number of Undo operations available to 99 and removed the preference from the Preferences dialog box. The lower setting was needed in the past when computers typically had far less RAM."