It has been on iPhones for quite some while, but on androids even longer. Before that it was in the form of some smart charging scheme that it would only finish charging until the moment it thought you would unplug it.

It makes a bit of a difference, but not dramatically: https://youtu.be/kLS5Cg_yNdM?t=3m26

In that experiment, it’s also unclear if the 30% lower limit or the 80% upper limit is more important. I suspect the former.

Anecdata, but I did this on my iPhone, and it did absolutely nothing for battery longevity compared to charging to 100% with "optimized charging" (which keeps it at 80% for as long as possible when charging overnight).

I charge my s25 to 80%. Previous phone (pixel) was also limited to 80%, but radio stopped working after 2 years so I had to buy a new phone.

Same for my s24, 80% battery limit and slow charging at night (most of my charging). It's been over 2 years and the battery seems to last just as long as day one