Naomi Wu has been talking about this for a while [0]. She lives in China, and has flagged this multiple times on Twitter, apparently without much response from the Signal team to her frustration. The issue seems to be that Signal simply sets the incognito flag when users are typing in the app, which kindly requests keyboard apps to not track the input. Unfortunately, since most American made keyboards for chinese writing are slow and difficult to use, most Chinese citizens use keyboards made by companies made in China, which does not provide much reassurance w.r.t adherence to the incognito request. Your concern seems to be spot on, and it'd be worth Signal at least making this clear to users when they first begin using the app.
While the answer from most technical users will be "just switch keyboards", it's also widely accepted that defaults matter and unless Signal makes an active effort to discourage people's default choice, the larger purpose of privacy will be somewhat defeated.
> You want to write Chinese, you need an IME. Apple/Microsoft/Linux/Google all have their own IME, but they aren't very good- typing on them is SLOW. Most Chinese use [...]
So, the ask is that Signal (a western company) develop a Chinese keyboard that is better than the vast majority of Chinese keyboards on the market (something Wu says is hard for western companies), and then force all Chinese users to use it, even if they prefer something else? What happens if they develop a bad one?
Keyboards aren't that special here. Signal opens up links in web browsers and some users install insecure web browser (also more commonly in China than in the US). Signal runs on an operating system and some users install insecure operating systems (also more commonly in China than in the US). There's no logical stopping point here until the whole device is Signal.
I think it's good for Signal to try to do one thing (encrypted communication) well, rather than trying to do everything.
While the answer from most technical users will be "just switch keyboards", it's also widely accepted that defaults matter and unless Signal makes an active effort to discourage people's default choice, the larger purpose of privacy will be somewhat defeated.
[0] https://twitter.com/RealSexyCyborg/status/119769534457579929...