Which kernel do you use on Debian? IIRC support for Intel Arc was added in 6.0 or higher. I am using Proxmox (based on Debian) and I had to upgrade from 5.15 to 6.2 kernel to get hardware decoding to work. Have you checked the Jellyfin manual? It’s pretty elaborate on how to get Intel QSV working.
Didn’t we reach a point where EULAs are non-enforcable? Or is that just in the EU? But regardless, Valve can just ban you and good luck doing anything about it.