Spigot & BungeeCord

Mojang recently released Minecraft 1.21 and the first builds of Spigot for this version are now available. As expected given the short time since 1.20.6, this release mainly just moves the experimental 1.20.6 features into the main server. There are however some underlying changes which you should keep an eye out for, particularly to teleportation between worlds which may have some as of yet undiscovered bugs.

Plugin Updates (updated)
With the exception of a targeted trial of enum conversions (see developer notes below), there are currently no significant API breakages since 1.20.6. Plugins using the 1.20.6 API should therefore largely function without change on this release. As always, plugins using the API should be preferred for this reason. If, however, you do experience issues relating to converted enums (Cat.Type, Frog.Variant, Villager.Type, Villager.Profession, MapCursor.Type, PatternType), the enum-compatibility-mode option in bukkit.yml may be of assistance.
As a server owner or developer please always consider the things you can do to increase API usage (eg, making feature or pull requests, preferring API-driven plugins). As we continue to work on making improvements to the API, there may be significant breakages of non-API plugins at any time.

Java Version
Mojang has decided to make Minecraft 1.21 require Java 21 or later. This is a Long-Term-Support (LTS) version and will be the recommended Java version until either a new LTS is released or Mojang requires an upgrade. For Linux users in particular, because this version is still quite new, you may need to install it from a third party such as Azul Zulu if it is not available in your package manager. Note this must be a JDK not JRE.

Backups
Please make sure you have recent and tested backups for your server before upgrading. As always it is not possible to downgrade your server to an earlier version.

World Upgrading
Although upgrades from between any two version pairs are technically possible, it is recommended that all upgrades are conducted from 1.20.6. This means that you should first upgrade your world completely on 1.20.6 using the --forceUpgrade server argument, and then do the same on 1.21.