throughout my quest to figure this out I've found out that MythicMobsExtension does in fact update. https://github.com/BerndiVader/MythicMobsExtension
last update was February 27, 2021.
So, while havent been able to get the custom mobs i desire to work fully (seems like the mobs themselves arent updated or im missing something) I can say the the plugin itself is still being updated and in working order