Google Assistant is now available on any device running Android 5.0 (Lollipop) and up, thanks to a trick found by Nikhilkumar038 of the XDA Developers forum. Essentially, the trick uses Nova Launcher's shortcut setting creation to trick your device into bringing you into the Google Assistant menu, where you were never meant to go. From there, you can activate Google Assistant, and it will replace Google Now On Tap for OK Google duties. The older alternative is still just a long press of the home button away. The tip is not just for Lollipop devices, of course; any device running a higher version of Android that doesn't have Google Assistant for any reason can use the same steps to get it up and running. There's even an alternative method involving Activity Launcher, for those who don't want to install Nova Launcher or are already using Activity Launcher. The best part is that it doesn't require root, and even works on more offbeat devices like tablets, so long as they have Google's app suite installed and the main Google App is up to date.
In Nova Launcher, long press an empty area of the screen, then tap "Widgets." In that menu, find Nova Launcher, then go into Activities. In that menu, find the Google App and give it a tap to expand available activities. You're looking for the one labelled "com.google.android.apps.gsa.staticplugins.opa.hq.OpaHqActivity," which should be the sixth from the bottom. Tap that, then tap on the Assistant icon that just appeared on your home screen. You want to go to the Your Stuff tab. If you happen to land on the Explore tab instead, just swipe over. From there, just try to add a reminder on that screen, and you'll end up being walked through the setup process for Google Assistant, including training the voice print.
For Activity Launcher users, tap on Recent activities, then All activities. From there, the steps mirror the Nova Launcher method; find the correct activity, tap it to add it to your home screen, then add a reminder to be whisked through the setup process. Once you've gone through either of these methods, you don't have to keep the apps you used anymore. Google Assistant is permanently installed and ready to use.