ne mozhno

[R. M. Cleminson]

Не можно is normal in 18th-century Russian (and earlier, of course).  Pushkin probably comes in just at the end of the period when it was current. (I am inclined to think that this is more relevant than the fact that the speaker in the example cited is Mazepa, a Ukrainian.)  Tsvetaeva is quite possibly evoking an earlier age when she uses it.  The reasons for its decline are obscure (to me, at least), but it is surely right to connect it with the concomitant decline of льзя, so that what we have in the modern language is essentially a suppletive pair, можно - нельзя.  This at least has the advantage of greater redundancy, so that one is less likely to mistake prohibition for permission, or vice versa.

Не можно is still possible in certain restricted contexts in the modern language, of which some have already been mentioned; another, not infrequently heard, is "не можно, а надо" -- "one not only may, one must".
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