Stable is a problem because a lot of these shells don’t offer any guarantees for breaking changes.
My own shell, https://github.com/lmorg/murex is committed to backwards compatibility but even here, there are occasional changes made that might break backwards compatibility. Though I do push back on such changes as much as possible, to the extent that most of my scripts from 5 years ago still run unmodified.
In fact I’ve started using some of those older scripts as part of the automated test suits to check against accidental breaking changes.
My own shell, https://github.com/lmorg/murex is committed to backwards compatibility but even here, there are occasional changes made that might break backwards compatibility. Though I do push back on such changes as much as possible, to the extent that most of my scripts from 5 years ago still run unmodified.
In fact I’ve started using some of those older scripts as part of the automated test suits to check against accidental breaking changes.