...but wanted to ask if sectional density is still a thing, as it relates to penetration.
And bullet design that addresses complication of passing through other materials first.
Yes, but it's just one ingredient to the recipe.
It sounds humorous, but long-rod penetration theory is the premise of military weaponry designed to penetrate.
That's basically what SD is...the higher the number the longer the bullet, thus in theory the more it will penetrate at a given velocity.
But in comes bullet construction, which is more critical to penetration. And then there's the materials/medium a bullet must pass through.
When comparing the same family of bullets, the SD will give you an idea as to what should penetrate deeper.
Oddly enough, there's a point in speed where the faster the projectile goes the less it penetrates. The reason is because of Newton's Third Law that says you'll receive as much force as you give. Eventually the force is so much that it destroys/deforms the bullet. handguns will never reach that speed....but still neat.
With handguns, makers want to simply control expansion and reach the FBI penetration depth at a certain velocity.
SD within a given velocity is the reason that a certain bullet within the same family within the same cartridge performs better than others.
Just a hypothetical example, many say a 124gr Speer Gold Dot in 9mm out performs the 147gr Speer Gold Dot in 9mm, or vise versa.
In it's simplest form, you need a mix of velocity and mass. One without the other does nothing, and too much of one decreases effectiveness.
Once you solve that ratio, you design a bullet intended for your purposes.