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I think a comma is necessary in such phrases because "that restaurant" is only one here:
A: Have you been to that restaurant, which is popular among young people?
B: Yes, I have. I enjoyed lunch there.
Can I write it without comma like this? Isn't it necessary to put commas before some certain relative clauses?
A: Have you been to that restaurant which is popular among young people?
B: Yes, I have. I enjoyed lunch there.
It depends. There are two types of relative clauses, restrictive and non-restrictive:
Restrictive
defines the meaning of a noun or noun phrase and provides necessary information about the noun in the sentence
e.g.
- The student who sits in the back of the room asks a lot of questions.
Non-restrictive
adds additional information to a sentence. A non-restrictive clause can be omitted from the sentence.
e.g.
- I want to thank my father, Mark Smith, for all of his love and support.
- I want to thank my father for all of his love and support.
Some information taken from Grammar: Relative, Restrictive, and Nonrestrictive Clauses.
Lexico states that:
You do not need to put a comma before restrictive relative clauses.
Kentlaw.edu states that:
Place proper punctuation around nonrestrictive clauses, but do not place punctuation around restrictive clauses.
In this case, your sentence is a non-restrictive clause and therefore needs proper punctuation, so it's best to place those commas.
Note that having the non-restrictive clause at the end of the sentence still requires a comma and period
