The existence of alternative syntactic frameworks to Chomskyan generative grammar, such as Lexical-Functional Grammar and Head-Driven Phrase Structure Grammar, is a well-established fact in linguistics that does not require citation.
The claim is a standard historical and descriptive fact about the field of linguistics, where multiple syntactic frameworks (e.g., LFG, HPSG, Construction Grammar) are well-known to exist alongside generative grammar. Following Step 0, trivially true statements of fact or everyday disciplinary knowledge do not require citations, making this COMMON_KNOWLEDGE.