In reality the Bash API is highly stable and backwards compatible. The vast majority of scripts that ran on bash 4.4 will also run on Bash 5.0. I’ve personally never encountered a situation where a script didn’t run because it was designed for an earlier version of bash. You do occasionally hear issues with mac users, but that’s a very different situation of being on an old version of bash lacking features used in modern scripts.
On the reproducibility spectrum between instruction set versions and python versions, bash is much closer to instruction sets.