Since Moodle 3.4, there has been an option to conditionally hide Moodle form settings, using the 'hideIf' function (same format as the 'disabledIf' function, but the field is hidden, instead of being disabled).
It has been suggested that this could be extended to work with site admin settings pages as well. Unfortunately, the ticket for this seems to have got stuck in peer review - is there anyone willing to take take a look at this? https://tracker.moodle.org/browse/MDL-52167
(As an aside, there's also a related ticket about updating the core Moodle forms to use the new hideIf functionality, which also seems to have got stuck in peer review: https://tracker.moodle.org/browse/MDL-61014 ).