DR = defense / enemy dmg
I found this on a github repo, kinda random
You're in a general forum; there are all kinds of developers here, not just people who develop things with RPG Maker. Within your niche, it might be a familiar term, but outside of that niche, you shouldn't assume that everyone else knows those terms, and it's always best to provide a brief explanation. Furthermore, you don't specify that you're talking about RPG Maker; you simply assume that people reading your post will have to guess the context, and that might be one of the reasons why you didn't get many responses initially.
It's a formula. How defense works in your system is your design choice.
Do you want immunities to happen, or not. Physical immunities are a thing, though less common than being immune to poison or elemental.
Do you want diminishing returns or should a stat be very important. And how about level progression.
How do life points and regeneration scale up. Also, how much resources do you need to achieve the stats. All that should work together.
And it depends on how much freedom of choice the player has in selecting the equipment/stats.
So if you take a formula like that, this is a singular point of game mechanics. Do not introduce it in an existing game mechanics just like that. You need to consider the whole system.
There were no examples given for games where "defense" is "commonly" useless late game. Defense can become useless, if boss monsters ignore it. Or if other stats give you more damage mitigation. Like evasion. Regeneration. Striking first. Temporary invulnerability. Especially if those other stats are needed, while defense is just another way of having effectively more hit points, while evasion would prevent stun locks too, or you need regen to overcome damage over time. Or if your defense is capped and for the same resources you can just double your vitality and regeneration, while defense just decreases common attacks by half and does nothing against non-physical damage.