I'm barely mentioning the features which people would generally expect out of chat apps.

Regarding the WYSIWYG, I always hear complaints, including myself that texts get weirdly formatted when they paste from somewhere else and it does more harm than good when symbols are automatically parsed, so the other option would apparently be WYSIWYG editor, which anyone will have no problem comprehending how to use.

You can't just expect everyone to read and learn formatting rules to type. I'm an engineer but I don't even want to remember some Mattermost formatting rules to avoid unintended transformations.