Do you mean stack-based source code languages like Forth or PostScript, or directly writing machine instructions for a stack-based ISA/VM like the JVM?
If the training set were loaded with Forth, LLMs would be knocking it out of the park.
In reasoning mode, if Forth were in high demand, I expect a modern LLM would blast out a python script to help it reason through the stack. That's how they seem to handle most problems that they don't innately understand.
That is what I thought initially as well, but I'm no longer sure that that is the case. Forth requires a mental model of what is going on on the stack and that mental model can get pretty complex. There is a lot more 'scope' in Forth than in most modern languages, you can reach pretty much any memory location and fetch any word from the stack, effectively the whole machine is in scope.