They're discouraged because they conflict with JSX. For instance, in the eslint documentation:
> Most codebases will want to enforce not using angle-bracket style because it conflicts with JSX syntax, and is confusing when paired with generic syntax.
I don’t think I’ve seen a single codebase using that pattern ever. With React being so popular and TS growing in popularity just about after React, people have settled on `as` assertions instead, whether they use JSX or not.
