When can a subjective relative pronoun and the verb "be" not be omitted in a sentence?

There have always been recommendations which are made to athletes.

I know "which are" can be omitted in the above sentence. Also, I learned that a subjective relative pronoun and the verb "be" cannot be omitted in some cases. Can you explain when they cannot be omitted?

Can a subjective relative pronoun and the verb “be” be omitted in the sentence below?

I have two sons who are doctors.

Answer

I can't give a complete answer. I have read that not all linguists accept the concept of "whiz-deletion". In "There have always been recommendations made to athletes", it is not obvious that "made to athletes" is derived from a relative clause "which are made to athletes"; an alternative interpretation is that "made to athletes" is a participial phrase that can by itself serve as a modifier.

You cannot replace "I have two sons who are doctors" with "I have two sons doctors."