The Signal desktop app does both too, I guess, but in a way that actually makes sense. Enter sends a message since IMs tend to be short one-liners. Shift-Enter inserts a line break.

But if you click an arrow on the top of the text box, it expands to more than half of the height of the window, and now Enter does a line break and Shift-Enter sends. Which makes a lot of sense because now you're in "message composer" / "word processor" mode.