They've been working on it for years, and improving it. They've only recently started claiming that their javascript rendering was ready for "prime time".

The "hashbang" URLs (they call them Escaped Fragment URLs) actually required you to use server-side rendering, that had nothing to do with the spider's javascript rendering engine. I used them on a few sites and it worked pretty well, but I have heard of mixed results on sites that weren't originally on hashbang URLs who transitioned to them. They don't recommend you use these anymore.