Hi I mained Soraka from level 7-30. I played Raka for about 5-7 hours a day pretty much every day for months on end. I played Soraka top I played Soraka mid, I played Soraka support, I played Soraka ADC, and I tried Soraka jungle a few times. Pre-rework I played Soraka top all the time. I own every Soraka skin and she's still one of my most played champions today.
post-rework you don't want to play her in a solo lane. I absolutely love her rework. I miss old Soraka but I love her rework. Soraka's Q has a 35% scaling ATM and her E has a 40% scaling (80% if they don't leave the E and get hit by both the cast and the snare) max level the base damage on both is 230. Her W active does not work anymore without an ally you can't have OP Raka top with a free 400hp heal that gave you 200 armor like you could pre-rework. Her passive only applies to allies too. Soraka mid is outdamaged by any other midlaner. Solo lane Soraka would be crushed by other solo lane supports like Lulu or Sona. Her healing is unreliable in a solo lane. Her wave clear denies her the ability to heal, her E doesn't apply damage to minions. I've seen a lot of Soraka mid, but I know if I play Soraka botlane I don't have to rely on the slowest bananas in the game to farm and I'll end up with more assists and more gold as a support Soraka than I will as a midlane Soraka. I could put a Brand mid and go support and my team will do so much better.
As for Leona, Leona is a secondary support. She was meant to be a toplaner, she's a primary tank. The only people who play Leona top/jungle are people who main Leona and pretty much solo play Leona top/jungle up from Bronze to Diamond. Everyone else plays her as a support because you can't use her passive otherwise. Someone like Braum can actually use his passive and the extra damage it gives without needing a lane partner, in my experience from playing Leona top, it's a lot of fun but your still not going to be as beneficial as an actual toplaner most of the time. Not when a Leona support lane should easily turn into a kill lane.
We could also mention other supports like Nami who the OP mentioned. When Nami first appeared mid they nerfed her waveclear because she pretty much had it all down in midlane. Nami's waveclear is pretty shitty now, her W can't hit 3 bounces without a gank from either side, she has a lot of pretty mediocre/low scalings.
Janna has a few okay scalings but her damage output is low and her damage output is a big buff on AD which makes no sense if you're playing her as a mage. As soon as Taric steps up toplane they nerf his pretty in pink self back down to unplayable as even a support.
You're B3 aka anything is viable. The amount of success I've had as Ashe supp with a Lulu adc (post the Lulu nerfs) in B3 is comical. There's still a lot of stupid stuff that's viable in low silver, but someone who is bad enough to lose to the new Soraka top/mid (if you've ever read the tips Riot posts for playing against Soraka the answer is 'focus her if she gets into range') should be streaming because they'd be top of the line B5 stream material.
Supports fall off super hard in solo lanes normally because they don't have the same DPS/Burst that is expected of a midlaner and your teamfight damage output is significantly lowered.