The State of Sona: V
TL;DR:
Patch 4.13 directly caused Sona to become unplayable, in the most literal sense.
Now for the long part:
Before patch 4.13, Sona was a sub-par champion that was marginally weaker than Nami, who was a strong pick at the time. A few players would still pick her, but Nami was the preferred choice. Then patch 4.13 was literally a collection of roughly 20+ documented nerfs and 5+ undocumented nerfs. I believe there was no more than 1 or 2 points that actually got buffed, but clearly receiving over 25 nerfs of varying sizes, some being severely kit-crushing major nerfs, far outweighs any minor positives. As a direct result of patch 4.13, Sona became completely unplayable in this game and this can be clearly evidenced in numerous ways, 1st and foremost being her complete disappearance from the professional scene. She is less likely to be played into a pro game than literally every other support and off-support in the entire game. Worse yet, despite already becoming the weakest support in the entire game, Rito then decided to nerf her even further, adding up to somewhere over 40+ direct and indirect nerfs stemming from Gutting 4.13 and onward.
Going into better detail, let's look at how severely Gutting 4.13 hurt her. Sona's damage output is the lowest out of any support character: she has only a single damage spell and it factually has lower base damage than any other support spell in the game. Additionally, with noteworthy exceptions to Thresh and Lulu, Sona is tied with Soraka for the lowest AD of the support class. Sona's full damage potential is altogether lower than any other support character in the game.
Then is her sustain: both the shittiest heal and shittiest shield in the entire game, their COMBINED value is barely on-par with normal heals and shields, yet it is also the ONLY spell of its kind(s) that lacks a bonus effect, yet sports the most expensive mana cost in spite of all these, while Sona herself has the 2nd-lowest mana of all mage supports. The shield is especially crap because its duration is less than half that of any other support shield, yet it's the larger HP value of the two halves. Aria of Perseverance is arguably the worst spell in all of League of Legends.
Next is the fact that Sona is literally the ONLY support in the game without reliable pre-6 CC. Hell, most off-supports at least have stronger+easily more-accessible slows than Sona, hence why she's even less likely to be picked into a pro game than an off-support. Even with her severe lack of CC, she doesn't have any real form of mobility and sits in the lowest tier of movement speed as well, whilst also being the squishiest character in this entire game. Killing a Sona is possibly easier than killing a caster minion, she's just as squishy as a caster if not more so, can't disengage nor retaliate. A Sona without Flash or ult is a dead Sona roughly 98% of the time. Speaking of her ult, despite being the least-nerfed part of her kit, it's one of the weakest ults of the support class, with its short range, small hitbox, slow channel and travel times, weak CC, short duration and long cooldown, Aphromoo himself stated all the way back at the end of season 3 that Sona should never be picked into a game that Annie is unbanned as Annie is just the superior ult-bot; that was long before Sona even became unplayable.
Let's not forget this next part: Sona's super-short shitty range. Nearly all of Sona's damage output requires her to be AT LEAST within 550 units of the enemy, where literally every single support and ADC in this game is capable of engaging or counter-harassing her. Considering the aforementioned fact that Sona has the lowest damage of the support class, she will ALWAYS lose a trade, or worse yet: since she's the squishiest character in the game with no mobility or reliable CC, in the case of an engage she is almost guaranteed to die without burning Flash. Hymn of Valor's primary-active targeting looks like this: Champions within 550 units > minions & monster within 700 units > champions within 700 units, so more often than not this spell requires her to be within AA range. Hymn's secondary active as well as Power Chord are both Basic Attack Modifiers, so both of these literally ALWAYS require her to be in AA range. As bad as that sounds, this isn't even the worst part: Sona is the ONLY mage support in the entire game with melee-range spells, of which she actually has not one, but THREE of which are her 3 secondary actives(which are NOT auras, that is just Rito's intentional mislabeling.) They are each 350-units, which is roughly the same as most melee spell and melee AA-modifiers, so yes they are in fact melee range. Since Sona has the THREE shortest-range spells of all mage supports, the 4th lowest is Soraka's W with a range of 550 units. Several other spells have 600 range and ALL other mage support spells are even higher than that.
Also take into mind the shitty design of the kit itself: both Sona herself as well as her passive especially are reliant on having spells of similar cooldown to each other, yet in spite of this necessity Fearless ruined that synergy by making those CDs very notably different from each other. This has directly fed the "spam Staccato" playstyle that fueled additional nerfs due to Sona's inability to actually use her other passives. Rather than fixing the problem, Fearless chose to punish Sona further for a situation that she's unable to do anything about. She literally received nerfs as a direct cause of receiving nerfs, typical Rito logic. Then is the problem of locking a LARGE amount of power into Ult ranks, in spite of being a support. Supports are supposed to be strong in the early game and will always be under-leveled; trapping power into Ult Ranks is inherently contradictory to the support class as a whole.
Lastly I need to discuss the current playstyle that was innovated by desperate players straining to be relevant on Sona. Of course there are her item choices, namely Spellblade and Echo: a pair of artificial damage sources. Sona herself is a character that lacks damage output, but to exponentiate this problem further, she has also the worst utility of all supports and off-supports in addition to the lowest damage output. Due to the strain of lacking a kit, players have innovated an artificial kit through item-purchases. Not to be mistake for buying AP, Sona actually has amongst the shittiest AP scalings of all supports, she has to build artificial bases that just happen to also give AP and AP scalings as an afterthought. Of course, these are expensive REAL mage items that are meant to be bought by a player farming minions and maybe obtaining kills, yet the support player will always have the lowest gold gain in the game. Due to the strain of being irrelevant without expensive items, players have innovated the method of pretending they are a carry champion: Sona's suggested playstyle is to never buy a Sightstone(it sets you behind in your item buys), farm and last-hit minions as if you are the ADC and intentionally last-hit all kills. That's not even an exaggeration, the suggested Sona playstyle is to intentionally troll your teammates, because your champion doesn't work otherwise; Sona herself doesn't work unless you're intentionally trolling. Picking Sona is literally synonymous to intentionally trolling.
or in other words, things that werent meant to go Support in the First place.