It is built into Android Messages. There used to be an option to enable it, but that is no longer there. You may get a message that enhanced services have been enabled at some point.
EDIT: A quick Google search found this for the V20: "Out of the box, the RCS capability, or "Enhanced Features" need to be enabled in settings. In the very near future, this option will be on by default and wont be visible."
Wasn't clear if that only applied to the V20 or all LG devices though.
So I decided to give it a try and disabled Google voice. A call was able to hand off from wifi to LTE and stayed connected well enough. Being able to use data while on the call was nice, speed tests and random app usage didn't seem to have any effect in call quality during my test call.
I forgot that hangouts won't do SMS anymore, so looked into trying Android messages. I thought I call rcs support for enhanced messaging being rolled out, but I don't see any options in the app to enable them. Does it work on the G6?