By default only players on the same Minecraft version as your server can join. With ViaVersion and related plugins, other versions can connect too. This needs Paper or a fork of Paper.

1. Open the Plugins tab in the panel (see Installing Plugins).
2. Search for and install ViaVersion. This lets newer clients join an older server.
3. Install ViaBackwards as well. This lets older clients join a newer server.
4. Optional: install ViaRewind if you want very old clients (1.7 and 1.8) to join.
5. Restart the server.

Some features and blocks do not exist on every version, so players on older clients may see replacement items. For plugin details, see the ViaVersion page.

If a player still sees an outdated server or client message, read Outdated Server or Client Errors.