It was not pre-quilted fabric. I quilted one-inch diagonal lines to hold two layers of the fabric together and the thinnest batting that I had which was made from recycled plastic bottles (Dream Green). Perhaps it wouldn't be as stiff if I had quilted it farther apart or used a different batting. I never thought of that.

No... I quilted two layers of the fabric together with Dream Green batting using one-inch diagonal lines.

It really is a lovely jacket. The extra quilting probably does add to the stiffness. We learn something from every project. I made a quilted jacket in a class years ago. The problem with mine is that I lined it with a slippery fabric, and it slips around on my shoulders. If I make another I will use something else for the lining.

Perhaps less quilting?
(of course, I don't know how much quilting you had in yours)

I did that too ... way back in the 80s!
And oddly enough, i was thinking just the same the other day .... of possibly, making another!

Though the benefit of the slippery fabric (polyester, I'm guessing?) is that you can slide your "dressed" arm in easy enough. Too cottony, and your clothing binds against the lining. Maybe stick with the slippery, and add in a false shoulder pad of cotton? Or fully across your shoulder width, the part you want it to sit still?
Back in early Eleanor Burns days, when I took a QIAD class, they had us use a poly-cotton backing, so that the quilt would slide easy enough at bed making time. In practical use, I wasn't so thrilled as I found it moved around too much while sleeping. And went back to cotton battings after that one! So I totally understand Dunster, what your predicament is!

Love your jacket! Great use for that beautiful fabric.

I've done a lot of wearable art, mostly jackets. I've learned that flannel often works well, but mostly I don't put any "batting" in them. I cut the pieces and quilt them to the backing on each piece and then put them together. They come out much nicer.

Was the fabric heavy or stiff to begin with? If it was, there's probably not much you can do about the stiffness since it didn't wash out.

It's not particularly stiff, but it is 65 percent polyester and 35 percent cotton.

That's a good idea to not put any batting in it at all.

I've never used batting because I didn't like the puffy look that you get from it. (Can I say adds 10 lbs? Who needs that?) Soft flannel is the most I use. With our weather, I don't use anything at all most times. Just quilt the top to a nice twill or interfaced cotton fabric. It works great.