Before Riot touched him, he was already one of the least played champs in the game due to his awkward playstyle, with only a handful of dedicated mains one-tricking him. These one-tricks put all their time and effort into mastering him, slightly inflating his win rate above 50%.
In response, Riot deleted a chunk of his health, exacerbating his nerf with the changes to the Celerity and Time Warp Tonic runes on which he relied. As a result, now even his dedicated mains can't win with him, and they've seemingly abandoned him; Sol has gone from being one of the least played champs on op.gg to being so unplayed that op.gg no longer has enough data for him to even provide any stats.
Riot has basically achieved what I thought was impossible: making a cosmic star-forging dragon-Thanos with a gorgeous aesthetic and charismatic voiceover into a champ no one is interested in.
Stat buffs are unlikely to rectify Sol's fundamental problem, which is a lack of options in his kit. So I think it might be time for a small gameplay update.
Personally I'd like to see his E make him untargetable for 1 second on cast (the cooldown on E could be increased by 5-10 seconds at all levels to compensate). This will give him another defence (outside of burning R) against his most frustrating weakness: being completely helpless when jumped on. Since the E's untargetability would be on such a long cooldown (65-45 seconds), it would still need to be managed extremely well, with the player being unable to use it for defence if they've used it to roam. So it wouldn't remove the weakness; just give prudent players the means to circumvent it with good decision-making, and provide another getaway against champs with multiple dashes (which even R is no defence against). Further, it would give the E more general utility, being able to be used with a quick cancel as an evasive manoeuvre in fights, and not just as a one dimensional roaming tool that becomes useless in the team-fighting stage of the game.