First, let’s clear the air. This is an official SNK product. It’s a fan-made flash game (now typically played via emulators like Ruffle or Flashpoint) created by Chinese developer Wing (or 闪翼工作室 - Shǎn Yì Studio) . The "5.5" refers to the core version of KOF Wing , while "Ultra Plus 2022" is a community-driven mod/hack that adds even more characters, stages, and balance tweaks.
The Ultimate Guide to The King Of Fighters Wing 5.5 Ultra Plus 2022