非限定關代 不能省略關係代名詞?

本句是以两個逗號區隔形成插入句，而插入句特性屬補充說明且刪除不影响句意，所以是非限定關代，這種用法很普遍且which is 需刪除但置於句尾則不可刪了，否則會語意不明。例The statue of liberty was originally a present from French people , which is located on a small island in New York Harbor.

When the clause with 2 commas separated to form the insertion clause=non-finite ="to delete" without affecting the meaning of sentence.
When "which is" is deleted, (Now that ) meaning time-reason-purpose-result-&manner- can be subsititued as an (Adverbial)
The (that) clause are not controlled by transitive verb=was
The function&meaning of the sentence
=The statue of liberty was originally a present from French people.
A finite verb form shows a particular time. "Am","was", "are", "will fall" are examples of finite verb forms, But "being" and " been" are not
eg"-The statue of liberty that witnessed the sailing ships NY "will fall"in the French Revolution 2018.
A non-finite verb does not show a particular tense or subject (=hence with or without this phrase in usage); and is either the infinitive or the participle form of the verb eg:-"located" in the phrase ""located" on a small island Harbor" is a non-finite participial phrase to be deleted.;
Since a non-finite verb does not show a particular tense or subject, then you don't misunderstand.

非限定關代是不能省略關係代名詞?
Yes, only the pronoun itself cannot be omitted.

According to the rules of reducing (簡化) relative clauses,
if a linking verb (verb-to-be) is used after a relative pronoun we can omit "relative pronoun + verb-to-be". The relative clause will then become an adjective phrase or a noun phrase, modifying or renaming the subject respectively.

1. adjective phrase
The statue of liberty, (which is) located on a small island in New York Harbor, is originally a present from French people.
** "located" is an adjective but not past participle.

The statue of liberty, (which is) covered with snow, is originally a present from French people.
** "covered" is an adjective.

2. noun phrase
The statue of liberty, (which is) the tallest statue in the world, is originally a present from French people.
This noun phrase is called appositive phrase 同位語.