One who depretes or depreciates (archaic); someone who reduces value or speaks ill of something.
From deprete (archaic form of depreciate or devalue) + '-er' (agent suffix). This is an obsolete word from Early Modern English.
Depreter is so old and rare that modern spell-check doesn't recognize it—it's a ghost word from when English was still deciding which Latin words to keep and which to abandon.
Complete word intelligence in one call. Free tier — 50 lookups/day.