long story short, the AmiKit X (10) was originally developed for ALICE laptop as a complete system. Since the laptop project is delayed, AmiKit X will be released even separately. But there's no update package that would update any previous version of AmiKit to v10. Therefore the 8.5 is the last update of AmiKit 8 edition.
(Btw. there's v8.5.1 too (also called Portable MK2), which is a special version adjusted to boot from USB flash disk. And AmiKit 9 for Mac is just a reloaded version of AmiKit 8 to work smoothly on Macs).