Websters Third International (1966) also has revocableness. But I too would favor revocability.

Das Random House Dict. von 1968 kennt revocableness auch

Right, revocableness is attested as far back as the 17th century, but it is now rare (which explains why it is not in the M-W). And thank Providence for that. What an atrocious mouthful.

... und seltsamerweise hat der Ngram Viewer überhaupt nichts dazu zu bieten ...

Looks like what Bryan Garner dubbed a needless variant. The entry could be changed to something like

revocability (also:) [rare] revocableness

But I think we would all say it would be better just to delete the entry. Definitely not a word that is, or should be, still in use.