> while web forums have had multiline text editors from the start so Ctrl+Enter=Send made the most sense

This is normal/standard behavior for multiline inputs such as the textarea, while input type=text/search/email will trigger a submit.

https://html.spec.whatwg.org/multipage/form-control-infrastr...

So basically a chat input based on web technology is saying:

"I'm a single line input, but will allow multiline editing if you ask nicely"