You can try and install it into 2017.2 but I can't guarantee Unity will play nice. The only problem you might encounter is that the prefabs will be broken. I include a demo to get you up and running but by no means are you FORCED to use it. I specifically isolated all WordPress functionality and GUI stuff from one another so that you can use any GUI system you want and create a GUI to suit your taste so, again, by no means are you being forced to use the prefab I include... but having said that, the thing is pretty darn comprehensive and even goes so far as to localize the entire login kit into 4 languages and auto detect the presense of additional assets and expose login preferences for them if found... If you have to create it from scratch you'd be losing out on a lot. Fortunately, the code will still be functional so you can easily recreate the entire thing by just dropping the scripts onto the right objects and dragging the right stuff to the right field. Easy... just very tedious and mind numbingly boring work... That is the only problem I predict might happen if you install the asset into 2017.2...
As far as the code is concerned, that will work just fine in any version of Unity that supports .NET4. So here is what happened...
I created the kits for Unity 4 and it worked perfectly fine since day 1, never requiring any fixes as new versions of Unity was released. The only reason to ever download a new version was to get access to new features. From Unity 4.... right up to 2017.2. Then came Unity 2017.3 and they broke the WWW class. Using Debug.Log you could clearly see they leave one of the mandatory headers empty but they still maintain "We didn't break it. We didn't", even though the facts are right there, staring everyone in the face... So what did this empty header mean for my kits? Simple: I can no longer contact the server. The server always returned an error because it wanted that header (the header tells it how long the header info is so since the header.length value was always blank WordPress couldn't process the request). So, yeah, no biggie... :( Sigh...
As it is, they said that the WWW class was just a wrapper around the UnityWebRequest class and eventually they want to get rid of the WWW class completely so I figured that since all my kits are now completely broken anyway, now would be a good time to change my code to work with UnityWebRequest and so I did that. At that point, though, since I was now forced to create a new version for 2017.3 and later I figured, well, since C#6 is as awesome as it is and 2017.3 supports it, why NOT update the code to require it? And so that came to pass...
Thus... the code now uses the alternate version of internet access, the version Unity prefers us to use, and thus it should work just fine in any version of Unity that has the UnityWebRequest class... BUT I made .NET4 a requirement so that is the only REAL restriction on what version of Unity you can use the CODE with. I am not sure what version of Unity they first introduced .NET4 in but I think it was 2017.1 so the code should work just fine in 2017.2
The ONLY problem here is that Unity does not allow you to install assets made in one version of Unity into another version of Unity that is older than the other one. For models and scripts etc it won't make much difference but for prefabs and materials they tend to break them completely. By completely I mean you will see the folder structure of the prefab in the hierarchy but all scripts on it will be missing and the entire thing is just.... well... broken.
As a general rule of thumb, if you get something in, say, 2017.3.15 and you try to install it into 2017.3.0 there is a huge probability that it will work just fine... but if you get something in 2017.3.0 and try to import it into 2017.2.x the chances are huge that it will break the prefabs. :( Unfortunately the only way to know for sure is to install it and see... and if push comes to shove then I can just send you screenshots of what the prefabs should look like and you can just recreate what you see from that... that is basically the worst case scenario I can imagine...
Nothing wrong with my code... just issues with importing .unityasset files into incompatible versions of Unity