Minecraft wiki entry for Combat_Update, associated text: 1.9.1 added elytra sound effects, the generic.armorToughness stat, and made the crosshair attack indicator visible by default, as well as other minor changes and bugfixes.
1.9.2 was released soon thereafter to fix a crash in 1.9.1, when riding boats and minecarts on Realms servers.
1.9.3 was released to fix several stability issues, add the /stopsound command, and to implement a blacklist for servers not in conformance with the Minecraft EULA.
1.9.4 was released soon thereafter to fix an erroneous Realms warning.