Adobe Camera Raw 9.1.1 expanded support for several popular cameras released around 2015:

Convert your new camera’s Raw files (.ARW, .CR3, .NEF) into format.

The primary driver for the continued search for a "free download" of this specific version is hardware and software incompatibility. As software advances, so do its system requirements. Modern versions of the Creative Cloud suite require 64-bit operating systems and substantial RAM and GPU resources. However, countless studios and hobbyists still rely on robust, older workstations running Windows 7 or older macOS versions that cannot support the latest updates. For these users, ACR 9.1.1 is the "final frontier"—the last version of the software compatible with their environment. It allows them to process modern files (within the limits of supported cameras) without the financial burden of upgrading an entire computer system.