Yea, it is possible either improvements to OTP or the changes made in Cowboy 2 make this moot. And if it does still exist it is also only a concern for a very very small number of users.
It just intrinsically feels it should be better that the decode code is in Erlang to keep consistent and low latency with increased concurrency – but I know, that doesn’t make it true 
As an Elli and Chatterbox user and maintainer (maintaining chatterbox being the painful one) I just wish Bandit was Erlang so I could consider moving on
)