If a Windows ISO claims to be “forever” free and not from Microsoft.com or a known OEM partner, it’s too good to be true – because it is.
The Windows 11 Pro Phoenix Game Edition, often abbreviated as RFISO (which might stand for "Retail Full ISO"), seems to be a customized version of Windows 11 Pro, optimized for gaming. The source you mentioned, ullversionforever.net , suggests it might be a site providing various versions of Windows and possibly other software. This report aims to provide insights into what such a version might entail, based on general knowledge about Windows editions and customizations for gaming.
Many “game editions” turn off Windows Defender, UAC (User Account Control), firewall, and automatic updates to improve performance. This leaves your system completely exposed to zero-day exploits.