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?