I think the dev team is very wary (and probably weary as well) of secret mage variants being playable in standard. In wild, secret mage has been around since ever, and I think there is a general hatred of the archetype.
This is evidenced by the fact that latest secrets they printed for mage are nearly strictly WORSE than effectively the same secrets for hunter. Thats astounding when you think of it. Hunter secrets cost 1 less mana (which is a HUGE deal for a low cost spell like secrets) AND they are more powerful (when comparing the latest ones.)
Comparison 1
https://hearthstone.blizzard.com/cards/102149-summoning-ward
vs
https://hearthstone.blizzard.com/cards/72340-emergency-maneuvers
The mage secret requires the minion you want to copy TO BE ALIVE the next turn, which is a nearly impossible feat in todays heartstone without something like STEALTH and DIVINE SHIELD, and even then there is a good chance your minion won’t stay alive.
Emergency Maneuvers is AT LEAST 2x as good if not 3-4x as good. It’s actually playable and a very strong secret when you have one very high value minion on the board.
Comparison 2
vs
Wandering monster is not even that strong a secret, but its probably 2x as strong as Vengeful Visage, because it can act as a 1-time damage shield from a very big minion and can save you against lethal. Vengeful Visage is at best a VERY WEAK tempo play and in reality, nearly everytime its played it translates to a 1/1 or 3/2 (at best) hitting your opponents face once before being wiped.
Comment
This secret is just SO terrible I don’t have words to describe it. There is no way to play this secret and EXPECT some kind of outcome. Generally you will get a random zero cost card, and in NEARLY every case this card will be useless or NEARLY useless. Not only that there is a very high chance this secret will trigger when you have 9 cards in your hand, burning (possibly) a VERY useful card from your deck.
If this card cost 0 mana, and was included in your hand for free at the start of the game, I don’t think this card would be worth playing. IT IS SO BAD. I’ve never seen a worse secret.
The best use of this secret is to use it and hope you confuse your opponent into checking for an Objection or counter-spell and thereby lose some tempo. THERE is no other good use for this secret, it’s actively a BAD card when you play it.