Signal team needs to address this.
“Signal should warn users who are likely using insecure IME apps”
Apparently Signal added a new support page for this: https://support.signal.org/hc/en-us/articles/360055276112-Incognito-Keyboard
Follow
“Signal Is A Good Start, But Beware”
https://www.efa.org.au/2021/01/22/signal-is-a-good-start-but-beware/
