This update switches to a new system to make the player invisible, namely the use of entity metadata. Previously, a simple invisibility potion effect was used. This caused some compatibility issues with other plugins however, so starting from this release a different method is used that does not require the use of potion effects.
This update also adds a warning message on startup if your server is running CraftBukkit instead of Spigot or Paper. CraftBukkit is very outdated and not supported.