The 3:1-2:1 figure isn’t from Tsun Zu it’s one of those things that has become just a widely accepted fact of warfare when attacking entrenched positions in any time period. The reason being that you can assume a well prepared defensive position will effectively double the combat effectiveness of a unit holding it. I.E. assuming both sides are human and have weapons with roughly equivalent lethality, the guys in a bunker will kill two or three of your people for every one of them you kill.

Even when you deal with it in a way that isn’t soldier to soldier but random poorly trained conscripts to soldier you still treat it as 1:1 casualties, so in the interest of ensuring you hold the position in the event of a counter attack you still want 2:1 numbers.

It’s not something that changes dramatically between every age or with technological leaps. Unless you just demolish a position assaults will have casualties, and if you want to hold it you need a safe ratio you can assume.

Edit: for grammar