As always in the world of third-party modifications:
First, a crucial clarification: was not a single client. Originally, "CMPack" referred to a curated collection of performance-enhancing mods, configuration files, and utility tweaks designed specifically for Minecraft 1.8.9 PvP. Over time, community members repackaged these mods into custom launcher versions, often bundled with OptiFine, 5zig, OldAnimations, and toggle-sprint mods. cmpack minecraft 18 pvp client 2021