Awesome to hear, im looking forwads to testing bugs found previously and seeing the new items avalble to craft with also
If you can play on 1.16.5, Drogo would imagine you won't be able to see or use any of the content that was added after that version.
I Would presume that, or the devs have a way for it to use old item assets like Diamond armor for it to overwrite Netherite Armor for example, but i would say that would get confusing(item wise). and for new mobs, i have no clue.
It’s just visuals and textures. If in 1.8.9 for example, your elytra is shown as a leather tunic but holds the name 1.9 Elytra