Well, it's in Google TV/Play Movies & TV app (since September) and Google Home app (since November). There's no standalone remote app.
Google home app also has the remote
It works really well as a button in the pulldown notification shade.
I believe the Google TV/Play Movie app on your Android TV also need to be up to date to sync with the version on the phone.
It's baked-in to the Google Home app now unfortunately. Personally The remote & design work better for me now than the old one did but I've read some people have had problems with it not working for them.
I still would've preferred if it stayed it's own standalone app. The Google Home app is enough of a cluster fuck as it is.
I think the Google TV app also has remote control built-in as well.