We have unearthed-arcana right now, which is just the current “brand name” for D&D 5e playtest material. Why not call a spade a spade and have a playtest-material tag for all our playtest material question needs?
Naturally unearthed-arcana would be a synonym, so it would still pop up for people looking for that tag name when writing questions, but what would show up after submitting would be playtest-material.
As a side benefit, this might also solve our old tagging problem with old D&D Next questions that can't be rewritten to be about the published D&D 5e rules, especially since the consensus that developed in that meta was that we should tag them distinctly somehow. I think playtest-material is the natural solution to that specific problem as well as to the general problem. We wouldn't go on a tagging spree for old Next questions, but playtest-material could be added to them reactively as they get noticed or bumped, slowly putting that difficulty to rest.
Other games occasionally see questions about playtest materials too. I think it would be useful to be able to distinctly tag questions about playtest materials regardless of game, for searching and future clarity. I think that would be an improvement over having brand-name tags for playtest material for different specific games.
I think playtest-materials would be useful and we should adopt it. Are there any thoughts on downsides that should stay our hands?