This is a community creation. There are no documents on confirming this is the right version.
Also this is a stub. The stub is used where you normally would use your paid FinalIK. This is not FinalIK it self. It’s more like a construct which helps you to preserve the data which needs to be bundled into your avatar to work in VRChat.
As you mentioned 1.9 is the required version to my knowledge. After buying FinalIK the oldest version you can download is 2.2 or 2.3 iirc.