one-dnd-playtest is not necessarily dnd-5e
We have a few old dnd-next-playtest questions, which became dnd-5e eventually. The systems used in both were quite different at times. one-dnddnd[-playtest] is just the next evolution of it on the way to dnd-6e or whatever they will assign as the next edition's name. So we should give it (or its playtest) its own tag.
In fact, I bet that in the future often the One-D&DD&D[-playtest] questions should not have either dnd-5e or unearthed-arcana tag, as they are an ecosystem on their own, even if in flux and development. Once we have a sufficient... branch from the previous edition, we also shouldn't tag it that way!