For those of us on Catalina beta already, we'll have to get a copy of the new launcher either via Steam or through direct download depending on account type. That's kind of a drag, because it means we have to wait for either Steam or the launcher download server to host the new patcher, whereas folks on Mojave will just get a patcher update. It will almost certainly take longer before the patcher update is available on Steam.
The above is going to be true no matter when the 64-bit launcher comes out. I'm not claiming it is coming today. Currently reinstalling ESO from Steam to see. The patch is released, because my Windows box is already patching.