If you are referring to one of the hidden menus, those have always been labeled as unsupported and could be changed or removed at any time. It sounds like it may have been altered/removed. I know of no way to restrict bitrates within the Roku itself. Depending on your Internet router, you may be able to add a restriction within it.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
Thanks for the answer @atc98092! That is correct. 'Secret screens' are not supported or intended for general use. These are development tools that may change or become non-functional at any time.
If you're looking to restrict data usage, you'd want to work with your ISP or wireless router manufacturer to make adjustments to your home network to help constrain data usage. You can also reach out to individual channels/services to see if they allow you to control these types of settings within their service. Some channels/services, such as Netflix, offer options directly within your account which allow you to control video quality settings to restrict data usage.