Currently, scrummaster is a canonical tag. However, other role tags are hyphenated like product-owner, product-manager, and project-sponsor. For various reasons, I can't create a synonym for it, and scrum-master would be more in line with the way the community has defined other tags over time anyway.
Redefining a tag requires diamond moderator privileges. Ideally, the scrummaster tag would just be redefined as hyphenated. In this case, I don't think we need to retain the unhyphenated version, but if we do it should be as a synonym of scrum-master and not the other way around.