Well yes, if you're using newer features, it's not going to be available on older systems that lack a newer bash version with those features available. I think that's pretty reasonable, otherwise we'd have to freeze the language and never add anything. But your older scripts will be very portable between future systems, and across different distros once they update. If you need to target an older system, you can't use newer features, but that's true of everything so I wouldn't expect any different from bash.