In Lr Classic 8.4 open the Preferences dialog and on the General tab uncheck the "Automatically check for updates" option. Close the Preferences, then close Lr. Relaunch Lr, open the Preferences and look and the "Automatically check for updates" option. I find that it is always checked.
I edited the Preferences file and found that
noAutomaticallyCheckUpdates = true/false
is always set correctly on closing Lr according to whether I have unchecked/checked the "Automatically check for updates" option in the Preferences dialog. The problem is when Lr is launched again, it seems that although the Preferences file option is being loaded, it is not being used to initialise the "Automatically check for updates" check box state.
I think it is an initialisation problem because unchecking the option, closing the Preferences dialog and then opening it again shows it unchecked. However, closing Lr when the option is unchecked and relaunching always shows the option as checked, even if internally the option is actually off.
Tony