i have no idea how magnezone exactly has 'better coverage' than jolteon when magnezone's only other notable offensive move is the great almighty signal beam while jolteon gets to shadow ball too in addition to that and if you're really desperate, double kick or rock smash. flash cannon only hits ice and rock types for SE but those 2 types aren't very common: you will likely not face any more ice types after Kyurem (except for Lapras which is neutral to flash cannon) while the most common rock types you will face is roggenrola's family which is always at least 2hko'd anyway.
also, remember that jolteon has instant utility in walling the hell out of elesa immediately because of volt absorb while magnezone most likely won't even be at lv25 by elesa and struggles against zebstrika because of flame charge. jolteon on the other hand can at least bite it to death while zebstrika is stuck to stomp because jolteon is immune to electric attacks.
honestly yes i feel magnezone is still the best electric type, but jolteon is definitely not 'severely' outclassed, because of volt absorb which allows it to tank electric stuff (granted though electricity is pretty much non-existent past chargestone anyway) but still it has quite a niche there. in fact, with its contribution to elesa i'm pretty much tempted to vote this thing high.
also regarding deerling i realised that it actually does have existent STAB once you get it: with the 10 free red shards you can tutor seed bomb onto it. yeah, can see this being high tier since now it cheeses clay.