I actually think changing the tag would be more likely to cause confusion. If someone comes here with a question and there are two different tags, they may pick airbus-a350 because that's the name they know, or they may be confused by the multiple tags to pick from.
To my knowledge, no examples of the original design were even built, and the design was abandoned almost a decade ago. The likelihood of us getting many (if any) questions on it is pretty low.
Furthermore, as fooot points out, we don't include variants in tag names. We don't have boeing-737-100, boeing-737-200, boeing-737-300, etc, or boeing-737-next-generation or boeing-737-max. We just have one boeing-737 tag. If the volume of questions ever warrants splitting them up, we can consider that then.
I don't see why the Airbus A350 should get special treatment in this matter.