In 1.20 mojang have added a new UI to the smithing table, and if your 1.16-1.19 server runs ViaBackwards, then 1.20+ players won't be able to open the smithing table, the same thing is also true with ViaVersion on 1.20+ servers and 1.16-1.19 clients.
The plugin also gives 1.7-1.15 clients the ability to open smithing tables.
Important:
AxSmithing won't let older clients on your server, you must download ViaVersion, and optionally ViaBackwards, ViaRewind
Customization
You can revert to the old 1.16-1.19 smithing table, that doesn't need a Netherite Upgrade Smithing Template
