The term is rather new, so there is very low probability for an established Latin translation.
Therefore I will coin a new one.
There are a number of words one could use to translate "to deny".
One is negare, but perhaps abiurare, "to deny any thing on oath", is a better fit in the present case.
As a corpus search shows, it can be used transitively.
Therefore something that can be denied on oath can be called abiurabilis, and the corresponding deniability is abiurabilitas.
Then it comes to finding a translation for "plausible".
Pitkäranta's dictionary suggests probabilis, credibilis, fide dignus, and veri similis.
The last option sounds a little off in this context, and judging by the entries for probabilis, credibilis, and fides leads me to think that fide dignus is the most appropriate choice here.
It might be translated as "deserving of belief", and that is what "plausible" is all about in this phrase.
Putting these together, my suggestion is abiurabilitas fide digna.
However, if you want a single word, then I recommend abiurabilitas, "deniability on oath".
I don't see how to add plausibility to the picture without adding a word.
If three words is too much, I suggest abiurabilitas probabilis.