Title of submission should be: 'Leaving spellcheck Enabled On Chrome Is a Privacy Risk'.

In the spirit of trying to do constructive discussion, the original post references the long dead Signal Chrome app. The new desktop garbage they push is a Node.js app called Signal Desktop. They use native spellchecking provided by Node and Hunspell (even on Windows, which could use native windows API available for 8 and newer and which probably is privacy violating)

To the best of my knowledge, Hunspell is privacy conscious software.

EDIT: Additional context, https://github.com/electron-userland/electron-spellchecker also confirms it only uses Hunspell on windows.