Based on the S9 Dev Blog:
The goal of S9 HP changes was to “Lessen the impacts of burst damage to allow for greater counterplay.”
Even if Hanzo had his one-shot for 250 hp, they would technically be still within the parameters of their goals for S9, as the impact of burst damage would still be net reduced, for both him and several heroes in the cast.
More heroes have HP over 250. These are the heroes that would survive a full charge 250 dmg headshot:
DPS
Bastion
Cassidy
Mei (won’t be able to HS+melee)
Reaper (won’t be able to HS+melee)
Symmetra
Torbjorn
Venture
Support
Life Weaver
Brigitte
Not only are there several heroes that he would still not be able one-shot, he would have to charge his arrows to full to hit the 250 critical threshold (Hanzo players used to partially charge their arrows to maximize RoF). His OHKs would be happening with much less frequency.
On top of this, Tanks have the -25% headshot passive. As a percentage Tanks would be taking much less damage than they did before, due to health increases as well as this passive.
Provided they do the correct thing in buffing his damage +5, nerfing Storm Arrows, and reducing his projectile size, he would be much weaker than he was in S8, and he was already by no means a top pick.