I asked the same question on the eu board 2 months ago. I did it because 1) as far as I can see you cant beat bronze soloQ as support and 2) several platinum e-heros keep mocking me for not being able to get out of bronze as support in soloQ.
Now I went from b4 to silver V playing Janna/Nami/Lulu going 46/37 55% winrate playing alone in flex.
Afterwards I played 110/101 matches alone in soloQ mainly playing Janna. At first I was playing at 57% winrate but after getting almost to silver and beaten back to b2 several times, I tilted so hard my winrate fell to 52% and I decided to quit playing support for good.
I have all the enchanters at mastery 7 and my Janna KDA of 214 ranked matches are 7.89:1
This was in the first 2 months of s8
On the eu forum I found 2 people that had beaten bronze alone + several that had done it with a doupartner. One playing Nami and one playing Zilean (and he said he felt he made it because of Zileans waveclear that is better than most supports, wich allowed him to impact the mid and late game macro more)
My theory is this: You cant get out of bronze playing support alone in soloQ, and probably you cant do it silver to gold eighter.
I personally feel the reason are 1) that with a bronze/silver skill ad carry you are tied to bot. Roam and bot falls and you carry dies horribly (and probably over and over) 2) you have no waveclear and can simply stand as a dumb rubberduck and look stupid while your team do NO wavemanagment. 3) people are less likely to listen to a support, so your macro knowledge means less when you play support.
I have spend the last 2 weeks playing ziggs bot (most fun I had in League ever tbh, I hope I can get good enough to play ranked ap carry and that it will keep being viable in s9) and not once have a macro suggestion from me been ignored (dive, dragon, baron, laneswitch whatever)
Now dont get me wrong. I am no e-hero. I am not even sure I could beat silver skillwise. I dont think "my team is holding me down" or that I "belong in higher elo". In fact I am a dumb noob.
But I am certain that ranking up from low bronze and to silver or gold is impossible unless you are a crazy smurf or a God or both. Also I am totally done listening to plats who doesnt even play support and last was legit silvers in s4 tell me that I am terribad because I cant rank up.