Huh. I had no idea that this feature was even available. I imagine that keeping an unlimited amount of undo data would take up a heck of a lot of space that wasn't actually needed, so it makes a lot of sense that they're switching it to premium accounts only.

To me, even 30 days seems generous. I suspect they have statistics on how often people undo changes, and from how far back. It'd be interesting to see the data that motivated the "30 day" rule, if it exists.