Ultimation
No one was "acting like" there are no "Secret Menus" in RokuOS, they were simply stating the menus have changed & there is no option to disable WiFi, and that Roku discourages/deletes references to accessing them (and I've yet to see a single user defend such a practice, because it's obviously anti-user and just dumb).
I think you are misleading yourself/using flawed WiFi/RF detection tools if you think the WiFi "auto-disables" after a period of time when using wired connectivity. Additionally, even if the WiFi were somehow disabled there is still other RF activity including BT (BlueTooth) within the device because the radio(s) within the chipsets/SoC would still be active, and even if those were disabled there would still be spurious/low-level RF/EMF due to the electrical activity within the device.
Its reasonable to want to disable WiFi or BT (specific wireless functions or protocols) though it would disable much of a device's functionality, and suggestions/feedback to do so have repeatedly been made: https://community.roku.com/t5/Suggest-a-Feature/bd-p/Wishlist