Discord's input is built on-top of Slate.js [1], which is sort of a more React-based alternative to ProseMirror. I've thought about building the same functionality on ProseMirror but I've always thought, inline decorations only work when you have a small number of marks. For example, you could have `~~==b==~~` be valid markdown, but not nice to look at.

[1] https://www.slatejs.org/examples/markdown-preview