world#getHighestBlockAt and world#getHighestBlockYAt both return the block above the highest block, including transparent blocks. Previously it only returned the highest fully opaque block.

The JavaDocs seems to indicate that this should not include transparent blocks:

https://hub.spigotmc.org/javadocs/spigot/org/bukkit/World.html#getHighestBlockAt-org.bukkit.Location-

"Gets the lowest block at the given Location such that the block and all blocks above it are transparent for lighting purposes."

https://hub.spigotmc.org/javadocs/spigot/org/bukkit/World.html#getHighestBlockYAt-org.bukkit.Location-

"Gets the y coordinate of the lowest block at the given Location such that the block and all blocks above it are transparent for lighting purposes."

In earlier versions (I believe 1.13.2 and below), these methods would return the highest full block, rather than the highest non-air block.

I have attached a simple plugin to test this. With the plugin installed, stand on any block and toggle shift to see the returned y value of both methods returned in chat.

I have tested this in 1.15.1, but I believe this change occurred in 1.14.