They supposedly added this as a opotion to 8.5 didn't find it tough.
You can do it in earlier Mayas by hooking up a undo for into time line change event. (this incidenttaly works in maya 8,5 too), This obviously only works for timeline time changes. And possibly the k key changes, if you type it as mel it'd be as real as anything else.
Search the forums ive posted this one before!
See time IS a node, so disabling this fully just suppresses the whole undo. This is the GREATEST feature of maya, having 3 time nodes in one scene is very very useful, (it allows me to reverse animations easily, re time them, drive them by other signals making the whole animation a sdk...). Then suddenly you'd start to have nodes that cant be undone.
Anyway you can udo selectively if you wish, you can also change undo to undo all time line changes in one go, and undo up to the last non time line change.
execute this (it gets permanent you can change it back in hotkey editor)
CODE
nameCommand -annotation "Undo Also Timeline Changes"
-c ("while (undoInfo -q -un==\"currentTime\")"+
"undo;"+
"undo;") undoTimeChangesAlso;
hotkey -k "z" -ctl -name "undoTimeChangesAlso";
youd need to change the undo in the menu too one more line code. Also if you wanted to undo the thing but not change time put a querry of time up fornt and then push it as current time as last.
