Mandarin speakers in some parts of China speak their Mandarin differently, so can't remember the exact pinyin. E.g. around Wuhan people can't remember whether a word ends with -ang or -an, similarly with -eng and -en. As a foreigner, I can't remember nu and nv, similarly lu and lv. Beijingers have trouble with -r endings.

I've regularly seen people use the handwriting IME in China. They were probably only post-PC computer literate, though :)

many older people like my dad, don't know pinyin well. he doesn't input texts, but he browses web sites. He uses a default home pages full of links to start exploring the web.