Design patterns are considered to be things that you can’t express directly in the language (that’s the Design Patterns book definition). Monads can definitely be expressed directly in Haskell. The only thing you can’t do is prove the Monad laws in Haskell (I think?).

Other languages, like e.g. Rust (because of lack of HKT), cannot express monads like Haskell can.

But yeah, you can definitely call it a design pattern if that’s what the culture around the language is like. But it would be like Java folks calling interfaces with default implementations for something goofy like “Interface with stateless implementations pattern”, and even they don’t go that far.