Everything was as he remembered it.
Can I delete it in the sentence above?
I feel it is weird to contain it in the sentence.
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It is not "weird" to include it, but it is not necessary.
An Ngram seems to show a recent rise in popularity of this type of omission:
Q: How was everything?
A: It was all as he remembered (it).
Can you see how it is a bit redundant here? What other pronoun or noun could sensibly be considered implicit and would drastically change the meaning of the sentence? The subject has already been stated once.
Most (if not all) fluent speakers of English would understand either way. It is up to you.
Click here to see the results of a Google Books search, which shows that both forms are in use.