I was unable to find out the rules here and what is the difference:
As yet, he has not received the package.
As yet little was known of the causes of the disease.
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That in turn means that in the two example sentences given:
it is the negative triggers (respectively, not and little) that license the use of yet.
We can easily see this from the fact that affirmative variations are ungrammatical:
Executive Summary: What a word -- especially a little word -- is supposed to mean has little to do with its syntax. Dictionaries are for words in isolation; grammars are for words in sentences.