It isn’t absolute. English is always evolving. What starts as two words may merge over time into a compound, or may not, or may exist in some grey area where either is acceptable. In the case of redshirt vs. red shirt, I think of the former as an individual concept, and the latter as a red individual concept. More specifically, the concept of a guy from Star Trek who gets regularly killed vs. a shirt that is red. But until a compound noun is widely understood and accepted, unless you’re getting graded on your writing, either would usually be acceptable. As a native speaker, I couldn’t even tell you if “hot dog” or “hotdog” is the more accepted spelling.

There are no absolutes in language, there are only reasonable deductions and guidelines. If enough people say something “wrong” enough times, it becomes “right”. Which is is why there’s a bunch of exceptions in languages. Best we can do is stick to making speech as unambiguous and as easy to understand as possible.