It may be an optional plug in. Extension never gained much traction so be cautious when sending to others.
In my versions of Ps, going back quite a bit, JPEG2K was an optional plug-in, and often located in a "Goodies" folder, on the installation disc.
You are correct that traction did not happen, BUT I think we might see a revival of it though, as Motion JPEG2K, as it is being explored by several companies as a Video CODEC. Maybe, or maybe not?
Same thing happened with Motion JPEG, with had all but died out. Then Nikon, and some other camera mfgrs. revived it, mostly for stills cameras, that also shot Video. Now, it's quite common. [However, many of those camera mfgrs. tweak the MJPEG CODEC, to suit their cameras, and that means that one might need the specific version, to edit the Video. Also, they play fast-n-loose with the Video wrapper, that contains there version of MJPEG. That causes great confusion in the Video editing community.]
Well, i don't know if it was a plugin (it was not installed by me, it was always there when i installed Photoshop), but until yesterday i had it available. Last night i did the update and the option to save the documents on this format disappeared.
About the caution, I just have a client that always ask's me to send it in JPEG 2000. It's a important client, so...
JPEG2000 is part of the DCI spec and used quite a bit, so it's actually kinda stupid for Adobe to not support it more widely.
Can, please, anyone that have the updated Photoshop CC, check if it's still there in that installation? That way i can see if it's my installation that is bugged or not. Thank you.
It is showing up in my Save As window. Mac with Maverick OS and all updates installed. I can't seem to insert an image by clicking on the camera. I get a big black window with insert image and a big red X. By the way, I am signed in.