Range of motion can be improved with training, but everyone has a limit. Where that limit is, depends on the person. If you practice every day for 6 months, then you can probably safely say that you've hit your limit.
I grew up where the Eastern toilet was the only available choice. Now, having lived in the west for over a decade, I struggle to squat on eastern toilets, often fall backwards!
Also, if your injuries involve tendons, ligaments or cartilage, you can feed those tissues with gelatine. You can buy plain gelatine and add hot water, stir vigorously and add a little apple juice for flavor and to cool it to drinking temperature. Consume rapidly before the goo starts solidifying in the cup.
I did this daily for a year or two to help heal old injuries. My impairment improved substantially and was never so bad again.
(For context, my left low back and my right heel both have lower range of motion from injuries. Presumably that can be improved.)
Update: I have a preliminary answer to my own question: squat more. I just read more hacker news while squatting, and the front of my ankles quickly became tired.