player knockback silently depended on the chunk packet broadcast system sending the player's motion back to itself, which broadcasting to hasSpawned will not do.
Grouping packets to batch by chunk instead of by player reduces bandwidth efficiency, because the number of active chunks is almost always larger than the number of active players.
With the old mechanism, a batch of packets for each active chunk (which could be dozens, or hundreds... or thousands) would be created once, and then sent to many players. This makes the compression load factor O(nActiveChunks). Broadcasting directly is simpler and changes the load factor to O(nActivePlayers), which usually means a smaller number of larger batches are created, achieving better compression ratios for approximately the same cost (the same amount of data is being compressed, just in a different way).
This change was made after exploring turning this into a permission. It occurred to me that this feature is entirely superfluous because it's non-vanilla, can be done by plugins, and is usually considered as a bug. In addition, disabling this behaviour required third party code just for this one thing because it was not able to be managed by a permissions plugin.
Instead, it's better to produce a plugin which implements this behaviour if it's desired, by making use of SignChangeEvent and PlayerChatEvent/PlayerCommandPreprocessEvent.
close#3856, close#2288
Not allowing this makes stuff like anvil damage, colour, wood type, live/dead bit, wet/dry etc all too much hassle to deal with.
Naturally I want to get rid of this shit altogether, but first it's necessary to construct a new system that we can shift into before all this bullshit can be addressed fully, so for now we have to work within the bounds of the old system.
This change will permit dynamic colours for concrete/concrete powder etc, dynamic wood types where the wood type isn't embedded in the legacy ID, and so forth. Allowing full flexibility requires either more old system hacks or completing the migration to a new system which doesn't have these limitations. I prefer to do the latter, but this change will make it somewhat easier to do.
