Thanks for your attention to this!
The post above suggest "legacy" as a replacement. I wonder if "backwards-compatible" would be more appropriate for some of our usages?
Edit: I realized "grandfathered" describes the object while "backwards-compatible" describes the subject, so sentences would need to be reworded for that replacement to work. For variables that are named "grandfathered" e.g. referring to a CSS spec, "legacy" may work better, though it's a bit more vague.