Shade 1 Jul
Well, ThunderBoard is not meant to be included in the core but it's just a "hack" to make it look like a single-page application. Implementing something similar in the core properly would mean a complete rewrite and possibly a switch towards modern frameworks, which is definitely out of reach for MyBB developers at the moment. If you want something that might work with proper routing and differential templating on a server-side basis, have a look at Flarum.

The stable release might not even come. Honestly, I don't know if I should put any more effort into ThunderBoard at all.