Backward compatibility does not imply aesthetic compatibility. Aesthetic behavior will be changed by almost any update to mIRC ever, since aesthetic changes are required to do simple things like:
1. Add new options to dialogs
2. Include new confirm dialogs
3. Create dialogs / UX for new functionality
4. Support new unicode characters (like, say, Emoji)
You cannot expect mIRC to have the same aesthetic throughout its evolution. As long as functionality remains the same, there's no issue with changing aesthetics, even of script output.
If you can provide an example where a script would follow a different code path due to this change, please raise it as a backward compatibility concern.
Your issue with /scid is fundamentally different from $calc (though I agree it wouldn't have a large impact) because changing the expectation of $1 being non-null would change (non-aesthetic) script behavior if a script was incorrectly relying on it. Adding precision to $calc() would not change (non-aesthetic) behavior, even if a script was doing something "incorrect" with the number.
