Learnable has been in use at least since the early 17th century.

By and large, when you find that an ordinary suffix has been attached to a word of the sort it is ordinarily attached to, and that the resulting sense is exactly that which such compounds ordinarily bear, it’s a legitimate word regardless of whether it appears in a dictionary. That’s what linguists mean when they say that a suffix or idiomatic pattern is ‘productive’: that it is regularly used to create valid new words or phrases. 

(And observe, please, that calling a term a neologism does not ‘invalidate’ it, whatever that might mean. To say a word is ‘valid’ can only mean that it is used and understood within its particular context or register, regardless of its age.)

In this case, -able is regularly attachable to any transitive VERB to express the meaning capable of being VERBed. Learn is a transitive verb, and learnable means capable of being learned. That’s all it takes.

Is "learnable" a valid English word?
What free tools are there to simulate RF circuits?
