Many players here play Crucible where you can get a blessing that will increase stun resistances.
Also crafting bonuses can give stun resistance (if crafted with right blacksmith) and those bonuses nevers shows in Grim Tools.
Stun definitely helps avoiding deaths,but 80% is not necessary.Some percentage like 50 or 60 is enough.Very rarely,if ever you’re gonna be stun locked.
In Crucible,blessing of Ulo increases all CC resistances other than slow.So my and other people builds tailored for 170 runs sometimes have like 25-35% stun.