2D billboards are certainly useful and common in game engines, no argument there. I think there are a couple challenges that make this idea difficult, though:
- While many game engines may have a clear concept of billboards, I’m not sure that authoring tools usually do. Looking an Blender tutorials there seem to be various combinations of constraints and particle systems used, and it’s not obvious how an exporter would identify what is meant to be a billboard when exporting.
- Rendering text is a complex thing all by itself – textures and mesh geometry don’t work well for text, so engines use more advanced methods that may be difficult to standardize.
- Billboards are often associated with LODs in a game. There have been several proposals for LOD systems on the glTF repository, and I think various developers have found systems that work for them using glTF’s custom extension mechanism, but there isn’t (yet) a clear consensus on one approach to LODs.
All that to say, it would require some research and design work to come up with a proposal that fits well into existing tools and solves the most common needs. If you just need a workaround for your own project, glTF custom extensions (see: About glTF extensions) are a good way to do this sort of thing within your own project, if you’re willing to do a bit of custom implementation in the tools you’re using. Custom extensions that gain traction with one or more vendors are sometimes a good stepping stone to an official extension.