GET /api/alpha/user was already returning default_sort_type but it now also returns default_comment_sort_type (for sorting comments, not posts). It was also returning show_nsfw and it now also returns show_nsfl.

You can also PUT those values to /api/alpha/user/save_user_settings to... guess what, save their settings.

Although PieFed has these default sorts it's up to the API user to use them when GETing /post/list and /comment/list as those endpoints will default to 'Hot' if there is no sort parameter.