1. You don't need closures at a language level to do event callbacks. Just the ability to refer to functions by reference and call those references. Closures certainly make it syntactically nicer, but you can work without them.

2. You don't need event callbacks in a webserver to make it performant. The memory/CPU overheads needed to make closures work are essentially identical to what's needed to make blocking threaded code work, as long as the threading is done in userspace and you have support for expanding stacks. Even if you don't have support for those niceties, modern kernels and hardware are perfectly capable of handling thousands of OS-level threads.