Spigot released a 1.13 build labelled with “Extreme caution” aka irreversible damage to worlds, they have outlined a 3 step process for when it would be a good time to test. So just keeping an eye on the Spigot news will give us a time-frame to when we can start testing.

https://www.spigotmc.org/threads/bukkit-craftbukkit-spigot-bungeecord-1-13-development-builds.328883/