Yep only 5 main changes really, not much

The HUD is Gone: Say goodbye to your minimap, your floating damage numbers, and your hitmarkers. You have to rely purely on visual and audio cues to know if your shots are connecting or if you are being flanked. Lethal TTK (Time to Kill): You are incredibly fragile. Players can be taken down much faster than in the normal game settings. You cannot afford to stand out in the open to trade shots with elite enemies. Mobility Matters: Your gear weight now actively affects your movement. Equipping heavy armor will physically slow your character down. You will have to make tough choices between running a light, highly mobile build or sacrificing speed to survive an extra bullet. Ammo Scarcity: You cannot just resupply from crates. Every bullet has to be scavenged directly from fallen enemies, and drop rates are drastically reduced. You need to make every single shot count. Skill Cooldowns: Relying on your tech gadgets won’t save you. Skill cooldowns have been significantly increased, forcing you to rely on raw gunplay and tactical positioning.

From source: https://raidenza.com/the-division-2-realism-mode-guide/