1) Arti is basically a prototype at this point, including it already would be reckless, 2) Arti is a client for the Tor protocol, and including it in the browse

1) Arti is basically a prototype at this point, including it already would be reckless, 2) Arti is a client for the Tor protocol, and including it in the browser wouldn't have any impact if JavaScript ships enabled/disabled by default in Tor Browser, 3) if you really want to, you can easily change the "Security Level" in Tor Browser to disable JavaScript for all websites by default. As an alternative for the last point, turn the Security Level to "Safest" or however it's worded, then use the included NoScript addon to enable it for just sites that just won't work without JavaScript. You get functional web + JS disabled in most places where you can.