I used to apply ALT+arrow shortcut to go back / forward when navigating folder structure. I've noticed that in DO12 it's not possible anymore to use this shortcut. Is there any way to get it back somehow?
It's not only still possible, it's still part of the default configuration:
Have you changed or turned off the File Display Toolbar? That is where the default hotkey comes from (it's tied to the toolbar's Back button).
If you edit or remove that toolbar, you might need to set up a separate hotkey to do the same thing.
The command the default button / hotkey runs is: Go BACK USEQUALKEYS
I've actually removed the folder navigation toolbar a long time ago and stick to the shortcuts.
The mentioned shortcuts were not present on my system somehow and I've added them. However they don't seem to work as expected - I keep landing in the same folder, e.g. I enter Windows folder, then press ALT + Left Arrow and I'm still in the Windows folder instead of C: drive...
UPDATE: I've actually figured it out - I've mirrored the text "Go back to %1" instead of putting "Go BACK USEQUALKEYS" as the function! 
When I changed it, it now works perfectly!