A lot of Sona's win-rates are probably due to how easy she is to play.
This video goes over top 10 win-rates of all time
And in doing so, points out some pretty interesting things about win-rates and champions that I at least didn't think of initially when contemplating what a good win-rate means.
The Warwick segment at 5:00 of the video goes over the most relevant thing to Sona's win rate though: in that an easy champion to play is pretty easy for anyone to pick up and play optimally without a whole lot of practice on that champion.
Sona's win-rate did go up after her rework and it's at an all-time high in modes like ARAM: but she's objectively a much worse support in regards to actually supporting. She's more of a...utility carry with a fairly big reliance on item scaling than she is a support like Thresh or Janna with next to no item reliance.
Higher AP scaling with lower bases and utility means Sona is going to be successful on newer support players that just build full AP without warding or grabbing sighstone, and she's also going to be more effective on a lot of Top/Mid/ADC/Jungle mains that would rather do damage and build full AP than support. Whether those players want to or not: Sona's kit is going to support her teammates with a more selfish AP/Damage-heavy build, and Sona's kit is always going to offer some level of support.
Where with doing something like running support Vel'Koz or support Brand: you can easily wind-up stealing a lot of CS, kills, bringing very little utility, and essentially throwing your carry under the buss so you can be a carry if you don't really know what you're doing with those champions.
However: Sona just flat out isn't played at the top level, and the last time she saw relevance within competitive play was largely in season 2. ATM Nami is just a better Sona with some skill. Like Nami though, Braum requires a fair bit of practice to reliably do better than Sona, as you can fail pretty hard with Sona and still do OK with her back-line healing and a reflexive ult, where failing with Braum especially can be much worse. If Braum gives up a kill early, he can often be to squishy to really play to his tanky front-line strengths for the rest of the game.
Anyways: I'd attribute Sona's respectable win-rates more due to damage-control and being an easy champion to do ok with for support secondaries than her actually being a powerful champion in her own right.
It's not a stretch to assume support secondaries make up the majority of support players given the current state of things, even in Plat+elo.
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