Export As PNG at 16-bits DOES NOT WORK in PS version 23.2.1

Save A Copy does it just fine here:

However, Export and Save For Web are both restricted to 8 bit, since they are intended for web/screen/mobile, where 16 bit makes no sense.

@Nancy OSheaYou are wrong in regards to PNG not supporting 16-bit images - unless I misunderstand your answer, PNG does support 2^16 tonal values in each colour channel  The following table is sourced from the PNG wikipedia page:

PNG actually does support full 16-bit PER CHANNEL images.

You are correct that terminology is one of the confusing factors here.

PNG8 generally refers to a 8bit indexed image - an image that saves a 0-255 values colour table, and is restricted to 256 colours. 2^8 = 256 colours. It is often said that PNG8 can only support 1bit transparency (either on or off), but the PNG specs actually allow for stacking a second colour table of transparency values, and PNG8 therefore can support full 8bit transparency. *see note below

PNG24 refers to a full colour PNG image, with 8bit per R, G, B channel. Each 8bit channel contains 0-255 values: 24bit = 2^24 (or R 2^8 * G 2^8, * B 2^8) = 16,777,216 colours. It lacks an alpha/transparency channel.

PNG32 refer to a full colour PNG image, with an additional 8bit alpha/transparency channel - so 2^24 for colour information, and 2^8 for transparency values.

When we talk about a 8bit, 16bit, or 32bit image, generally the number of bits PER CHANNEL are referred to. That means 2^8, 2^16, and 2^32 values per colour channel.

So which image file formats support 2^16 values per channel? Correct, PNG is part of that list. What is more, it can even include a full 2^16 transparency/alpha channel. So in fact it supports MORE than 16bit images: 16bit per channel + a 16bit alpha channel.

PNG does not support 32bit per channel, however.

Image editors tend to divide web export from saving an image in a certain specific file format. Therefore the 16bit image export is generally found in a completely different place in the user interface compared to PNG8,16,32 export for web purposes. In Photoshop, as @J453 mentioned, to export a 16bit image as PNG we have to work in 16bit image mode, and then export via File-->Save As (PNG).

(Technically it is impossible to save a 16bit full range image in Photoshop, and it will effectively be downgraded to 15bit.**)

Finally, PNG also supports PNG1, PNG2, PNG4, PNG5, PNG6, and PNG7 indexed tables: 2,4,8,16,32,64, and 128 colour limits. This may crunch image files sizes further. Unfortunately, the new Photoshop Export As dialog does not expose these to the user. The older legacy web export dialog does, however. But neither allow for that second table of transparency values when using these or PNG8.

*Note: Photoshop does not support this. Fireworks did/does, as do various PNG optimization tools. Older versions of PS CC seemed like they supported this, but the 8bit png was actually exported as a 32bit one, according to one of the developers. Refer to the conversation here:

https://community.adobe.com/t5/photoshop-ecosystem-bugs/export-as-8-bit-bug/idi-p/12772630#comments

I pointed this out to the developer in question, and he is looking into whether it is possible (hopefully) to implement this).

** A major caveat of Photoshop's legacy code core from the nineties is that its 16bit image mode cannot handle more than 15bit plus one value. This means it is impossible to export a full range 16bit PNG image from Photoshop. In practice the number of colour values are halved from 65,536 to 32,769 2^15+1 values.
Luckily, Photoshop is the only image editor on the market with this issue, so it is possible to work around the problem by switching to a different editor when circumstances require this (HDR effects workflows, scientific purposes, astronomy, CGI).

Saving as a copy does not work. Everytime I try to make my file 16 bit it automatically goes back to 8 bit no matter what I do with my files it just always wants to go back. I work in gradients and there's color bands appearing because of this in many of my photos. Please have the team continue to work on this thanks!