Shield Gating is a mechanic that was introduced to Warframe with the 27.2.0 update. It is designed to cut down on the number of one-shot deaths that players suffer in the game. During endgame activities, as you push deeper and deeper into a run, enemies will scale up, and the screen readability can drop. This means you are more likely to get hit by something and instantly die to it, such as a grenade. This can be particularly annoying in Arbitrations, where that instant death will end your run. Shield Gating has been implemented to try to reduce the chances of this occurring.
Shield Gating for players
Shield Gating means that as long you have active shields, a shot that would deplete your shields will not also do damage to your health. When any shields are active, an incoming hit that depletes your last bit of Shields will not continue into your Health pool, and also triggers a brief time where your Health is protected. Once that period is over, you can take Health damage normally. Additionally, you will no longer take Slash Status Effect damage to your Health while Shields are up.
So, if you get hit by a grenade that, in the old system, would instantly kill you, instead, the damage will stop when your shields are fully depleted, and you will get a brief moment of invulnerability. One that period ends, the next shot you take can kill you if it does enough damage to your health.
Shield Gating for enemies
Enemies will also have the advantage of Shield Gating, but there will be skill-checks to get around it. If you hit a shielded enemy with a headshot, or in a weak point, then you can completely bypass the Shield Gate. Hitting them anywhere else will cause 5 percent of the damage you have done to bypass the Shield Gate. This allows powerful builds and weapons to skill feel very effective at lower levels, while skilled use of accurate weapons will nullify enemy Shield Gating at higher levels.
Slash Status Effects will now deal damage over time to Shields, Toxin damage remains as-is, bypassing Shields to affect enemy Health directly. Damage from Warframe Abilities will ignore the enemy Shield Gate.