This was never meant to be a complete solution to back pressure in Erlang and has recently been remove for the next major release: https://github.com/erlang/otp/commit/2e601a2efc19d64ed0628a5...

I’ve heard speculation that it really only helped scheduler efficiency and this didn’t apply all that well on SMP schedulers and has finally been removed since non-SMP builds are no longer supported.

As for Pony, their actor model doesn’t support selectively receiving messages so their queue handling may have some different trades to make there, as well as interaction with primitives like promises (which are used where Erlang would prefer a ref tagged receive clause). Spooneybarger has done work on backpressure in Pony so maybe he or other Wallaroo labs people can elaborate.