No. What is not used as a relative pronoun. There are sociolects that relax this rule, but they are stigmatized. What does occur as a fused relative marker, meaning that which; e.g, What doesn't kill you strengthens you.
– John LawlerApr 07 '20 at 15:48
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The relative "what" that occurs in fused NPs like "I really liked [what she wrote]" is a pronoun.
– BillJApr 07 '20 at 16:20
@JohnLawler What is used as a relative pronoun, but only with a suppressed antecedent — With joy beyond what victory bestows.
– Anton ShepelevSep 21 '22 at 15:48
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That's the fused relative pronoun that @BillJ and I mentioned above. You can always substitute the phrase that which for this use of what. That's why it's called "fused".
– John LawlerSep 21 '22 at 17:22