bsInstanceof is a switch-like structural directive that branches based on the runtime class of a value. Each *bsInstanceofCase matches an instanceof check and exposes the narrowed instance to its template; *bsInstanceofDefault renders when no case matches.
instanceof directly in templates. However, it's not sure if the feature will land in version 21.2 - it may be introduced in version 22, which is usually planned to release new experimental features. (commit)