The StimulusJS handbook has an automatic content refresh example at https://stimulusjs.org/handbook/working-with-external-resour....
As for NEW MAGIC, DHH mentioned it will have a Rails server side aspect to it so I don't think we can compare these tools directly.
He did say other server side frameworks could implement similar behavior too but it would involve the framework having built-in support for websockets and job queues. This is based on a question I just asked him on Twitter at https://twitter.com/dhh/status/1334953167327547397.
So it sounds like NEW MAGIC will probably make it super easy to push / broadcast stuff from the server to the client, but to get everything working with a really nice API and it being seamless will require a substantial amount of work if you planned to re-invent it in another framework.
Maybe for basic partial page updates where you don't need to broadcast something to every connected client over websockets then Turbolinks 6 will be enough on it own. If that ends up being the case that's amazing because it'll be fully client side.
In any case the near future is looking to be the best its ever been for developing very nice feeling apps without going the SPA route with any back-end technology.
I've been following them since the intercooler days and they've been great (functionally) although maybe a little drowned out by all the SPA chatter. Still a great library IMO.