Digging straight down is a speedrun strategy of last resort for modern versions. The typical progression to enter the Nether is to start by getting 7 iron (3 for an iron pick, 3 for a bucket, 1 for a flint and steel). You want to get that, grab some food, and be in the nether at around 3 minutes.

Doing that by digging down is not really feasible. There are two main routes.

In the village route, you spawn next do a village with a blacksmith. You get some iron or an iron pick from the blacksmith, kill the iron golem for the remaining iron, grab some wheat and then find a nearby lava pool or completable ruined portal. There are variations where you can trade with a villager for a bucket, but by then most speedrunners would have already reset.

The second and more typical modern route is using ocean. You locate a shipwreck which gives you iron and food. Then you find a ravine that has magma blocks (indicating there's lava underneath) and use that to build your portal. To my knowledge this isn't viable in 1.18's world gen though.

When speedrunners do go underground in the overworld, they'll usually try to find a cave first to find exposed iron and lava. Digging down is the last choice.

After doing the basics (stone tools, safe base, light and food), I'd always do that as soon as possible. Diamond stuff is very useful for later parts of the game, and you'll need a lot of redstone to build interesting circuits.

It's its own form of exploration, at least in the versions with fun cave generation: you always end up in some cave complex that you can then explore. But I'm happy to see that it's less needed now, because interesting caves are more commonly accessible from aboveground.

I've been playing since the release of beta and once it was understood in what layers diamond spawned more frequently it became very common. I've seen some people refuse to do it just because it kinda kills the wandering exploration vibes.

It's called strip/branch mining (https://minecraft.fandom.com/wiki/Tutorials/Mining#Branch_Mi...) by players.

Everyone (including children) learns this method over time, be it by checking tutorials online, reading a wiki, or just naturally due to how simple and efficient it is.

You just had to mine straight down to y 8/13 (this changed in the new patch), make a long corridor and then just branch out of that corridor in lines.

At some point every player goes "Where, oh where in the world is the goddamned iron I need?", they google it, find ores spawn differently at certain heights and go from there.

Speedrunners don't do that, for one. They don't generally go searching for diamonds, only grabbing them if it's in an easy location like a chest.

It's also what I figured out when I was a kid.