While Mega Salamence is certainly a powerful attacker with enough bulk to be extremely annoying if taken on head-first, I'm not sure it warrants a ban. To start with, a number of common, strong attackers handily beat it:
-1 252+ Atk Huge Power Mega Mawile Play Rough vs. 0 HP / 4 Def Mega Salamence: 296-350 (89.4 - 105.7%) -- 37.5% chance to OHKO
-1 252+ Atk Huge Power Mega Mawile Sucker Punch vs. 0 HP / 4 Def Mega Salamence: 88-104 (26.5 - 31.4%) -- guaranteed 4HKO
0 SpA Mega Salamence Fire Blast vs. 252 HP / 0 SpD Mega Mawile: 192-228 (63.1 - 75%) -- guaranteed 2HKO (-1 Eq does even less, and setting up a dragon dance is pretty much gg).
4 SpA Teravolt Kyurem-B Ice Beam vs. 0 HP / 0- SpD Mega Salamence: 552-652 (166.7 - 196.9%) -- guaranteed OHKO (The choice scarf set is very common from what I've found. The rare Haban Berry Kyurem-B also wins this matchup with ease, and the somewhat gimmicky Bulky Weakness Policy set will live an outrage and KO back. Similarly, Weakness Policy Aegislash can OHKO with Shadow Ball at +2, though very good plays by the Mega Salamence player can sometimes lead to it winning).
-1 252+ Atk Mold Breaker Mega Gyarados Ice Fang vs. 0 HP / 4 Def Mega Salamence: 188-224 (56.7 - 67.6%) -- guaranteed 2HKO
-1 252 Atk Aerilate Mega Salamence Return vs. 0 HP / 4 Def Mega Gyarados: 145-172 (43.8 - 51.9%) -- 14.1% chance to 2HKO (Ok, so this one's a bit worse. For one thing most M-Gyara run outrage instead, however I find Ice Fang to be very nice as it takes care of Mega Mence while using mold breaker to destroy Dragonite. It's also an 86% chance of winning which isn't perfect, but plenty good enough TBH).
-1 252+ Atk Choice Band Dragonite Outrage vs. 0 HP / 4 Def Mega Salamence: 350-414 (105.7 - 125%) -- guaranteed OHKO (Banded Dragonite is far superior to the Scarf set you posted about, getting a number of key KOs which are very nice. Alternatively, a DD set could win the matchup as well though I feel it'd be a waste of Dnite).
252 Atk Aerilate Mega Salamence Return vs. 0 HP / 4 Def Meloetta: 291-343 (85.3 - 100.5%) -- 6.3% chance to OHKO
252 SpA Choice Specs Meloetta Hyper Beam vs. 0 HP / 0 SpD Mega Salamence: 397-468 (119.9 - 141.3%) -- guaranteed OHKO) -- guaranteed OHKO (
Dream Eater Gengar 's infamous Specs Meloetta can happily defeat Mega Salamence, while being somewhat hard to predict due to a Perish Song/Trick Room Meloetta set also existing It does lose to the rare Sub set, however this is very uncommon).
-1 44+ Atk Choice Band Rhyperior Rock Wrecker vs. 0 HP / 4 Def Mega Salamence: 390-458 (117.8 - 138.3%) -- guaranteed OHKO
252 Atk Mega Salamence Outrage vs. 212 HP / 252 Def Rhyperior: 141-166 (33.2 - 39.1%) -- 100% chance to 3HKO (Another of DEG's sets, Choice Band Rhyperior can beat a huge number of physical attackers while taking almost no damage in return. Note that Earthquake does even less damage than outrage. The rare substitute set gets utterly destroyed by Rock Blast, another of the moves often on this set, which can 2HKO in any case).
Moreover, there are multiple ways of beating Mega Salamence without straight-up attacking it harder. For example:
252 Atk Aerilate Mega Salamence Return vs. 4 HP / 252+ Def Eviolite Chansey: 313-369 (48.7 - 57.4%) -- 94.5% chance to 2HKO (Personally I prefer putting 252 into HP and relying on its naturally high SpD stat to beat special attackers, however with charm and seismic toss the only way you're going to lose this one is a crit or the rare substitute set).
Mega Aggron and similar CounterCoat stratagies can work extremely effectively against Mega Salamence, especially those that can deal with its setup. For example, Mega Aggron running taunt will have pretty much a guaranteed win against Mega Salamence, with the rare Fire Blast still only causing a 3HKO meaning even if it is used, the Mega Salamence player will have to play very well in order to win still.
Sets not running Fire Coverage can also lose to Ferrothorn, with Leech Seed + Protect recovering back plenty from an unboosted return while Gyro Ball punishes any setup it may consider. It's far from a perfect counter, but has a good chance of winning if used well.
Other, more gimmicky sets such as Sub/Disable/Encore Alakazam can also be considered, although a good player will be able to beat these without too much trouble.
tl;dr: There are many ways of beating Mega Salamence in the current 1v1 metagame, and while it's certainly a very strong physical attacker I wouldn't say it's any more intimidating than Kyurem-B and is somewhat more predictable. It's certainly something you need to consider when building a team, but there are multiple threats that need to be considered just as carefully, such as Mega Mawile, and I would not argue that these are broken but rather defining parts of the metagame. A suspect test would be ok, however while it's perhaps the biggest threat in the current metagame I don't think it is at all close to requiring a ban.
