This checkbox never should have existed. It has caused nothing but confusion and bugs. What is stable or not is defined by semver (the presence of a prerelease ID makes a package unstable. All others are stable). The stable flag was just some denormalized metadata we stored for searching and filtering so we wouldn't have to reparse the version string every time. When a user saves a version, use the SemanticVersion library to decide if the version is stable or not. Here is the function to use:
CommandBox uses the semver library as well and never sees or uses the stable flag in the ForgeBox DB so letting people mark arbitrary versions as not stable just causes confusion.