I'd say it's more a combination than just a singular issue . . .
The eyes were nice for slot efficiency, but with the huge nerfs to trinkets, it feels like season 1 again but without manual wards to buy. Seriously the nerf to the blue & yellow trinkets were nonsensical; blue was barely used, and yellow had the option of getting a free pink ward every few minutes instead of a normal ward every minute. Then blue had it's vision range cut noticeably and yellow normal wards nerfed to only last half the time; vision has been massively cut down in total.
I miss the old gold generating items; philosopher's stone & heart of gold were generalist items, but simple & effective without being overloaded or overpowered. Riot removed then in season 3, then when they finally noticed supports behind on gold they increased gold generation for the lowest member of the team by 1gold every 5 seconds. While a good idea, it's not nearly noticeable in practice. They added other gold items that are largely situational, and depending on the matchup, there will not be a good nor effective option. The coin gives a few gold whenever an enemy lane minion is killed within a short distance; the shield occasionally duplicates the gold given from a lane minion for your closest ally when you kill the lane minion; and the spelltheif generates gold occasionally when you poke an enemy champion.
Good ideas in theory but what about the edge cases where none of the 3 modern gold items are beneficial? Say you've got the shield and your carry is someone with incredible wave-clear, you'll be lucky to get once use from it per wave. How about the coin, if your opponents are good zoning champions, you're only going to get a couple minions each wave, which is still a small fraction of an actual minion bounty. Then the spelltheif, if your opponent can out-range you, how do you win a trade against them when you can't even poke them to keep up in gold?
These specialized gold generators can be better likewise under specific circumstances, but much worse when unable to profit from them at all. Riot removed the choice between 'consistency' & 'reward/risk' by forcing the later.
-I agree I liked the herald more as it wasn't severely build dependent, since near every carry paired with a support is physical, and thus benefit from the life-steal aura. And as you said it's replacement was obsolete almost immediately; when there were requests to remove critical hits, but riot enforced it everywhere!
-There was also the banner of command, changed so that the buffed minion is immune to magic damage; entirely useless when riot changed the meta to be near entirely physical damage.
-Then what about the ardent censer? Not only nerfed; but like the zekes was made irrelevant, as there's plenty of cheap attack speed options for ranged physical carries now.
-However there's still other supportive options, though they're clearly made for other classes with a price tag to match; frozen heart for tanks, crystal scepter for mages, ext.
Besides struggling to ward, as if being items behind wasn't crippling enough; levels are also the common issue. Since riot's 'great' idea of making games faster via snowballing, supports fall behind faster & harder. Not to mention the often overlooked removed mastery that was meant to combat this issue, generating some exp constantly while around a higher level ally. While minimal, it was better than nothing; even when your team is snowballing across the board, the winning support is still a level or two under the loosing carries. What is often forgotten is stats per level; as the fat assassin trends show, a level advantage is enough to to win a stat-check instead of items.
Those were just the direct nerfs, there's still an indirect nerf most players didn't notice. Assist sprees & shutdowns. Assist sprees unlike regular sprees increased gold gains from consecutive assists without dying; which was removed in preseason 6. Shutdown gold used to just give the bonus gold to who-ever killed them, but like assist sprees was changed without consent for implications; shutdown gold is now divided to among all players of the team evenly. Sounds good in concept, except for the rate & cap; when an enemy kills 3 times or more without dying they give you a bounty based on how many they killed, 100 gold per kill, with a cap of 5 kills. Which was & still is negligible when snowballing is in effect; before an enemy could go 10 kills but only gives a bonus 500 gold when killed, now an enemy can go 10 kills but you only gain 100 gold when taken down. Shutdown gold doesn't remotely return a fighting chance, when a bulldozing enemy is finally circumvented.
The removal of assist sprees gutted supports, but the change of shutdown gold made the game even more 1-sided than before.
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Long story short; supports have been shafted with gutted & useless items, game mechanics changed to put them behind in gold & levels; yet riot is oblivious why no one wants to play them.
Sona's highest winrate build: 
were all a step in the right direction, and anymore slot efficiency we can bring to the role would help. Some support items are in the works with the mage update, and this may give some help to the class. One problem is that Riot kinda doesnt care "consistently" about the class. Zekes Harald was removed for Zekes Harbinger, and Z. Harbinger was a good item... in season 5. Though I actually did like Z. Harald, Harbinger is used less now than Harald was when they removed it for not being used. The problem is twofold, first; matches are so stompy and fast that building an item to help your ADC get kills (in a narrow window) is always a risk over improving your own survivability/utility, second; Many ADCs get more than 50% crit, not all but 
or 
are used in the game and they go up to 70% crit. This means the Z. Harbingers moment of power giving 50% crit is slightly wasted, and that doesnt feel good. Z. Harbinger needs a cost reduction to 2,200 gold to get people to buy it at this point. That or it needs to go back closer to its release charge stacking values to charge up faster.
