For some, the context may include only the original 28” motorbikes, from the teens and twenties; for others perhaps also 26” balloon tire motorbikes with the double drop bar frame; many manufacturers continued the use of the term (or similar) for their semi-streamlined or streamlined bicycles with curved tubes.
I would call the arch bar bikes - curved truss frame roadsters, yet include them in the greater motorbike “era” (even though they were closer to the ToC for the original designer).