technically "IE mode" still exists inside Edge for old enterprise sites that never got updated, so it's not 100% dead even now.
microsoft finally put it out of its misery in 2022 and replaced it with Edge (which is just chromium under the hood now anyway). IE was the punchline of every web dev joke for like a decade because of how bad it was with modern web standards.
IE6 specifically is basically a running joke in web dev history, companies were stuck supporting it for YEARS after it should've died because of enterprise software dependencies.