This is part of the 1.13 update (Pre-Release 1). You can read more about it here.

For SubServers.Client, alot of the changes are related to the new Item IDs. Those of you on 1.12 or lower will still be able to use IDs like 38:4, but those of us that will be on 1.13 or above will have to use the new IDs (for example, minecraft:red_tulip). Also, TitleManager is no longer required to see title messages on servers 1.11 or above.