I am afraid the following message won't reach everyone.
So, I will try again.

Dear everyone,
First of all, I would like to say thank you to those who have responded to
my question as to the "difference between 'as' and 'which'."

And, I am very pleased and interested to find that your reports include
what is new to me (perhaps most of the Japanese learners).

In Japan, no grammar book explains the difference between "as" and
"which".
Nevertheless, we sometimes come across such entrance exams as

You carried the luggage yourself, (     ) was not necessary.
a. as     b. which    c. ...    d. ...
(Choose the right one)
Of course, there is no explanation why "as" is inadequate even in a book
which includes this question.

What is beyond my expectation and therefore very interesting to me is
that everyone agrees that all but 1-b are relevant.

> 1-a. You carried the luggage yourself, which was not necessary.
> 1-b. You carried the luggage yourself, as was not necessary.
> 2-a. He is a foreigner, which is evident from his accent.
> 2-b. He is a foreigner, as is evident from his accent.
> 3-a. Her feet were bare, which was the custom in those days.
> 3-b. Her feet were bare, as was the custom in those days.
> 4-a. She has married again, which was natural.
> 4-b. She has married again, as was natural.
> 5-a. Tom was not at home, which is often the case with him.
> 5-b. Tom was not at home, as is often the case with him.

I have expected "as" can only be used in fixed phrases such as "as is
often the case",
and that in these fixed phrases, "which" cannot be used.
So, I have expected the reports that 3-b is too formal and not so natural
while 5-a is
somewhat odd.

I am especially fascinated by the  reports that acceptability depends on
whether "not" is in the relative clause or not.

Thank you.