AArch64 is the 64 bit version of the ARM architecture, as published by ARM Holdings. It has gained much headway in recent years, being used in many mobile phones. It also is gaining market share in the server world, due to performance advancements and its power management. It also is starting to gain steam in the personal computer world, as Apple in 2020 started using it for most of its Macs. This chip, the Apple M1 chip, has performance comparable to Intel and AMD processors.
The primary barrier to OSDev on AArch64 is the lack of standardization among peripherals; even once you've settled on a board to target, you may not have much documentation on how to program it.
The situation is improving, as ARM SystemReady is a fairly new standard that aims to standardize ARM hardware. It allows for OSes like Windows, Linux, and the BSDs to be ran out of box on compliant hardware. This means that implementing AArch64 support in your hobby OS may be a good thing, as its popularity and usage continue to expand.