As @stschoen mentioned above, there is an actually unlimited undo history, but as soon as you close the patch, the history is wiped clean and the damage is done, so it is best to make a backup copy in advance. Personally, I have begun taking modular components of patches that I find useful and think I will use again, and copy them out of the patch I am building so I can paste each singularly in a new blank patch and save that in my library, like any other thing you will see if you open the A3 library (those options that you see categorized were all once just part of a series of nodes that composed a patch, and modules were singled out as useful and made into the library we all know and love today ) Then when I open the app each day, I try to always remember to start with a blank slate, and quickly build up, using the singular modules in my library as building blocks.
That is my work around, and it takes a lot of conscious effort (for me) to not open a completed patch and alter it as ideas occur to me, as I have destroyed and had to redo hours worth of work due to my own stupidity because of this easy misstep. As mentioned above, I am one of those that requested a ‘read-only mode’ for patches to offer write protection and save me from myself, but this will not be possible until A4 comes out. I hope my contribution to your inquiry helps a bit. Happy patching, and welcome to the community!