I'm not sure if this is the correct department to nerf him in for several reasons.
Firstly, Yasuo's defensive abilities work differently than Riven's. Riot has already tried making damage stick better on Yasuo by nerfing his passive shield.
His shield has already been gutted since release. The shield value was decreased to around 2/3 of its original value and eventually received a tiny buff when Riot realized it didn't even block half an ADC's auto late game.
The shield regeneration on his E has also been completely removed. Yasuo used to be able to get his shield back around every 5 seconds like Riven, but now Yasuo needs 13 seconds of straight walking to fill his Flow Bar from 0 to 100 at level 18 with tier 2 boots and shiv (Or if he uses Last Breath of course).
Just recently, Yasuo's shield regeneration took another hit in exchange for 50 to 100 more hit points. It actually takes him 17 seconds now at early levels to recharge his shield.
Also, don't forget the fact that he can't choose when his shield activates. Most champions have no problem throwing a ranged auto at it before engaging, and melee champions can punch right through it.
I am mentioning his passive Flow Shield because it essentially acts very similar to hp/5. The only thing the shield nerfs accomplished was that they significantly decreased the number of Top Yasuo games.
Secondly, Yasuo mitigates damage by mostly avoiding it completely. He is extremely effective against skill shot mages, but struggles significantly against champions with point and click attacks such as Azir, Malzahar, and Annie. Nerfing his hp/5 will only further emphasize his strengths at dodging abilities and expose his weakness to those he can't dodge.
On top of that, Yasuo also has some of the lowest base HP and MR in the game.
Given his win rates on Champion.gg, statistically skill-shot based mages such as Brand and Vel'koz can absolutely demolish Yasuo because they can land skill shots consistently enough. Between a 17 second uncontrollable passive shield, 25 second Wind Wall, and some low base stats, it seems Yasuo does struggle against some skill shot mages.
If you want to nerf Yasuo, consider making his double critical chance passive kick in later by tying it to level or something. In terms of defense, Yasuo is possible one of the worst champions in League.