Undo should be enabled by default.

[taviso]

I think modern users will be surprised that you have to enable undo (/Worksheet Global Default Other Undo Enable Update).

[multizone-uk]

This was originally because Undo used too much memory and people often had sheets which consumed all the memory available (especially in MS-DOS). You could even get Lotus-Intel-Microsoft expanded memory cards to let you page more memory in and out for gigantic sheets. - Angus