How to parse 'as yet'?

Question

7. as yet [as conj. 25b]   =    a. Hitherto, up to this time, = 3;  with a negative = 4;
implying expectation or recognized possibility of coming change.

yet = II. Senses relating to time.
2.a. (a) Implying continuance from a previous time up to and at the present (or some stated) time: Now as until now (or then as until then): = still adv. 4a. Often also implying contrast to a future or subsequent state more emphatically expressed by as yet (7a) [...]

Please help me dig deeper than definition 7 above, which I already understand and ask NOT about.

Question 1: OED pinpointed the right definitions of yet (an adverb, adjective, or conjunction) in as yet, but it doesn't identify the lexical category. So which lexical category is yet?

Question 2: In as yet, I can't even determine whether as is an adverb or conjunction; so I don't know where to start on OED's entry on as. Which OED definition of as matches? Please identify.

Afterword to user 'rogermue' 's answer: Idioms can defy the principle of compositionality, but Wikipedia attests to success in interpreting prepositions separately in some syntagma.
So even if as yet is an idiom, I wish to persevere to dig deeper.

Answer 1

The issue is as yet undecided.

"as yet" idiom, meaning up to now.

It is no use trying to understand the meaning from the two words "as" and "yet". The expression has its secrets and I doubt that anyone can explain how it came into being with this sense. Acoustically it reminds me strongly of German "bis jetzt" (until now), but of course, that is no explanation.

There is a similar mysterious expression with "as": as from today/as of today, meaning from today onwards.