This varies by configuration. Some aircraft and airlines do not.

Only one out of the last 25 or so flights I’ve been on have an outlet.

For example, Southwest is one of the worlds largest airlines and didn’t have outlets on any of its 700+ airplanes. (Neither AC nor USB) They just announced they will start ordering jets with USB ports starting this year.

Also it is fairly common for short haul or regional jets to lack power outlets as well.

Surprising to hear that such a large airline wouldn't have power outlets in 2023 but perhaps it makes sense, because Southwest doesn't really do "long flights". Good info for fliers to be aware of.

For Southwest and short haul fliers, I would then recommend an USB battery bank. They come in large capacities now which would cover several full charges.

I no longer have USB-A, which is the only USB receptacle I’ve ever seen on a plane.

They make USB-A to USB-C adapters.

True, but if I'm traveling I'm not likely to bring a second phone charger cable just for this, especially because they can't charge as fast as USB-C cables. I bring a USB-C brick for my computer and USB-C to lightning cable for my iPhone.