I've only ever had to support armeabi v7a, so are there any devices that require I support this architecture? I'm using the latest version of Xamarin forms btw.
I'm going to qualify that I'm not familiar with Xamarin but should apply to Android in general.
No, you don't need to support arm64-v8a. You only need to add support for arm64-v8a if you need specific JNI libraries for performance, usually for games or image processing tasks.
2019 Update The Google Play Store is now requiring 64 bit ARM support and providing a timeline
The Android Compatibility Definition Document for 7.1 - Section 3.3.2 32bit Native ARM Compatibility states that instructions deprecated in 64 bit ARM must be available to the 32 bit ARM code.
What that means is that Google/Android is mandating compatibility with 32 bit ARM code with any ARM64 CPU device, so any code compiled now will run on future devices without any CPU architecture issue (depreciated Android APIs is a different question).
More detail about supported Android CPU architectures can be found here: