No, this will not work on Java 1.8.9.
With Minecraft changing all the time, it's nice to have a way to go back to previous versions of the game. Java 1.12.2 is the modding version, and Java 1.8.9 is known for its PvP playerbase. Unfortunately, things do change, and so many newer features won't work with older versions of Minecraft, this custom item pack included. This page will go over all Minecraft versions that it is compatible with.
One thing to note is that if you are using the most recent version of Minecraft, you can use this custom item pack. This was made with recent versions in mind, so if you're running the current version of Minecraft, you're good.
Also note that this page refers to build/release versions of Minecraft, not the two different versions themselves. If you're not sure if you have Java of Bedrock Minecraft, click here.
The Java version of Minecraft is the older version of Minecraft. When people talk about Minecraft, you can assume they're referring to the Java version and 90% of the time you'll be correct in your assumption. Java Minecraft is split into its different releases, and each one, as well as custom versions, can be individually installed and launched from the Minecraft launcher. Java features servers such as Hypixel, and generally has higher quality mods made for it, however many aspects of the game are locked to specific versions.
The Bedrock version of Minecraft is a newer version of Minecraft initially made for mobile devices under the name "Pocket Edition". It's still commonly referred to as Minecraft PE today, but Bedrock has been coined as a term that covers this version of the game on all of its supported platforms. Bedrock is notable for its easy-to-use multiplayer and friend features due to Microsoft account integration, as well as its ability to emulate older features on newer versions, ensuring that unless it's an unintentional glitch, nothing on your world will break after an update.
J.0
J.0.1
J.1
J.2
J.2.1
J.2.2
J.2.3
J.3
J.3.1
J.3.2
J.4
J.5
J.5.1
J.6
1.16
1.16
1.16
1.17
1.17
1.17
1.17
1.17
1.17
1.17
1.17
1.19
1.19
1.21
B.0
B.0.1
B.1
B.1.1
B.2
1.18.30
1.18.30
1.18.30
1.18.30
1.19.50