As part of the documentation improvements, a new design proposal guideline  is proposed to replace and simplify the current process. The basic idea is to encourage developers to share their design via mailing list and tech forum, without a well-formatted design document. If developers prefer a document for a new feature, please provide an integration guide/user manual instead.
Please check the details in the patch below.  https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/14104
The rendered html version: https://ci-builds.trustedfirmware.org/static-files/KTOoRMhB4VsgNQFkdMgWZcZGn...
Best regards, Hu Ziji