What would happen in a 1.12 player on a 1.16 server got a higher-version item?

I'm going to be starting a Minecraft server, and I intend to allow all versions from 1.16.5 - 1.12.2.
Like Hypixel.
However, I thought of something before doing that. The server will run 1.16, meaning the nether will be generated using the new 1.16 generation. If a 1.12 player were to join, would their client crash, or would the nether just be bugged for them (update blocks from old PE)?
What about items, too? Would those become bugged too?

It would crash. When the client sees something it wouldn't recognize - blocks, items, entities, etc. - it just says "I'm confused" and will crash. It's like if you loaded a modded world without the mod.

Some servers have specialized plugins to port 1.16 blocks and items to older versions such as 1.12, like replacing basalt with glazed terracotta, or replacing netherite gear with dyed leather armor, but normally it is best to host your server on older versions if you want to port many versions of Minecraft to your server.