How to disable high contrast text when that option isn't available?
I finally found the solution. A computer with ADB is required. Attempting to do this from a local terminal will fail with a permissions error: "Failed calling service settings: Failed transaction (2147483646)"

Prerequisites:


ADB enabled in developer tools.
The last option in developer tools, "Disable permission monitoring" I believe it's
called (on my Spanish phone it's "Desactivar supervisión de
permisos"), enabled. If not then the command will fail with the message


  Security exception: Permission denial: writing to settings requires:android.permission.WRITE_SECURE_SETTINGS



Then, with the phone connected to the computer, simply execute:

adb shell settings put secure high_text_contrast_enabled 0

Thanks to:


ADB+Accessibility
I can't enable WRITE_SECURE_SETTINGS for an app over ADB