They're offering a text file. Presumably with an If-modified-since header, although it's hard to check now. There is no approach you can describe that doesn't

They're offering a text file. Presumably with an If-modified-since header, although it's hard to check now. There is no approach you can describe that doesn't run afoul of the described badly-behaved browser app which willfully retrieves the entire file afresh at every init. If it can be downloaded, it will be downloaded directly by the badly-behaved mobile apps.